A string of arrests since the Jimmy Savile scandal may mean we are guilty of judging the past by the standards of the present

It's never the right day to make this point. But many people are now being arrested over allegations of varying seriousness which took place a long time ago. Not because new DNA evidence has come to light, a welcome instrument of belated justice, but because deplorable behaviour, once ignored or tolerated, is now viewed in a more severe light. Is there an unhealthy element of retrospective justice to all this? It's that which troubles me.

This week we learned that the comedian Jimmy Tarbuck, 73, had been arrested in connection with unspecified child sex abuse in the 1970s. Last week another telly star, Stuart Hall, admitted a string of offences from that era, shortly after Rolf Harris, 83, emerged as the hitherto unidentified "household name" whose collar has also been felt.

At the weekend the Sunday papers had a field day over the arrest of Nigel Evans MP, deputy speaker of the Commons, on suspicion of a comparatively recent rape and sexual assault, between July 2009 and March 2013. Evans emphatically denies all and says he remained on good pub terms with his alleged victims, two young men in their 20s, until these accusations. His friends and – more important – his constituents have rallied round. At times like this you learn what people think of you.

It all seems odd, but the what and whys of sexual behaviour are never easy to judge. We can all be naive, complacent, complicit, cynical or suspicious. Remember just how indignant ("preposterous") the squalid, boastful Hall, 83, sounded just a few weeks ago? What's now clear is that people around the BBC during Hall's It's A Knockout celebrity knew about his predatory sexual behaviour towards young women.

As with the Jimmy Savile affair and Operation Yewtree's clutch of celeb arrests, some suits at the BBC connived in systemic misconduct, the exploitative abuse of celebrity power. It was still evident when last year's Panorama expose was shelved to protect a bogus "tribute to Jimmy" show at Christmas. Cover-ups are ugly, but not confined to licence-payer-funded TV.

I haven't finished yet. The current arrest-fest is not all about sex. There's an obvious parallels with Operation Weeting – the police investigation into tabloid phone hacking – and Operation Elveden, the one investigating tabloid bribery of coppers and other public officials. I say "tabloid" because what used to be "broadsheets" did not (to my knowledge) do either, except in rare public interest cases.

Trials are pending over Yewtree and uncertainty hangs over others arrested. Convictions over bribery have occurred. At least 64 journalists of varying age and seniority have been arrested. The Guardian's Amelia Hill was interviewed under caution in connection with leaks from Weeting about the squad's arrests, but not prosecuted.

Getting information is what newspapers do. So all papers involved protested loudly at what they saw – see – as intimidatory police attempts to gain the upper hand in a post-Leveson world. Are the papers just an interest group special pleading? Yes and no. Remember the subplot here, the shape of the new press regulatory regime, politicians' royal charter model v the press's own model: deadline 22 May.

But part of the press campaign is to make a fuss about assorted secret courts and secret justice. Issues range from Ken Clarke's "secret courts bill" and the furtive court of protection to routine secrecy in the family division of the high court. Here some shocking decisions are also taken in secret, sometimes on the say-so of hard-to-challenge social work reports.

Stung by challenges to their honesty and competence – Hillsborough, the miners' strike, the phone-hacking cover-up – the police seem to be trying to shift a few goalposts to tighten their control over procedures and publicity. They do so with the help of naive suggestions in Lord Justice Leveson's report and public antipathy to press excesses.

Result? The other day they tried not to name Paul Andrew Greaves, a retired copper actually charged with stealing £113,000 from Warwickshire police HQ, not merely arrested. Acpo, the police chiefs lobby, wants it to be routine not to name people merely arrested, a policy rejected by civil liberty lobbies and newspapers, which argue, rightly I think, that publicity given to Hall's arrest encouraged other victims to speak up.

The papers are determined to keep everything as open as possible, arrests, charges, courts. I'm with them. It's a good principle, but open to abuse, often in collusion with coppers motivated by operational needs (good), self-promotion (it's only human) or tip money (human, but naughty). Think of the monstering of the murder suspect Christopher Jefferies. I suspect Evans might be getting one now were it not for the Leveson stand-off.

But hang on, there. If ex-PC Greaves's blushes were spared, why was the Evans arrest all over the papers anyway? The papers tell their readers it wasn't a police leak and politicians are fair game in the anti-politics era, though they're not always guilty.

Remember how Damian Green (now the police minister) was arrested and his Commons office turned over on very dubious grounds? How Andrew Mitchell lost his job as chief whip – he's suing the Sun – after what now looks like an old-fashioned police/tabloid stitch-up? Remember how coppers leaked like rusty buckets over their doomed investigation into Tony Blair's alleged cash-for-peerages? Cops and tabloids have since fallen out very publicly and are fighting to regain purchase on the system. No wonder some of us were initially unsympathetic when tabloid heavies got the dawn arrest stuff they relished for other people.

But that was then. By mid-2013 it seems to be getting out of hand in some cases, a power battle turned into a witch-hunt solemnly dressed up as justice. What should bother us is that all these cases concern activities, often long ago and widely known at the time, deplored by decent people but connived at by the less fastidious. Screwing the fans? Sex pests in the office, in Fleet Street as well as the BBC? Yes. Bribing coppers? Hacking phones? Creepy "uncles" with a fondness for the kiddies? Yep.

None of it was confined to politics or the media in the decades after the "sexual revolution" of the 60s which my colleague Stuart Jeffries describes with admirable scepticism here.

That's a justification for airing past sins, so that victims of abuse who covered their own shame and confusion for decades, sometimes with fearful consequences, can at last be heard. It's a therapeutic process for some. From a feminist perspective, Suzanne Moore made that case powerfully here. It makes for uncomfortable reading. Except that it's not the full story. It wasn't all sexploitation, as our own victim culture now assumes.

An old thespian I chatted to in the pub the other day recalled the hordes of teenage groupies wanting to screw those pop stars at the BBC of his youth, elsewhere too. Yes, there were clearly victims of the predators squad, but not all were. What's more, established and influential female actors of a certain age would press their affections on young male ones too, said my old thespian. Crazy times which only Phillip Larkin seems to have missed, if you believe the myth (don't).

Society and its values change. Each generation deplores the cruel callousness and lack of sensibility in the generations that went before it, though each generation – including this one – makes cruel and callous judgments of its own. It's not as if we are making such a great job of sexual relations or protecting of our children from sexploitation, is it?

So where to draw the line under the past? When Israeli agents snatched the Holocaust organiser Adolf Eichmann from Buenos Aires, I thought that fair game: he was a big bad fish. Likewise Klaus Barbie, the Butcher of Lyons, extradited to France from Bolivia by changing politics in 1983.

But when Margaret Thatcher insisted on pushing through the War Crimes Act of 1991 to catch a few small fish on old and fragile evidence I was dismayed – as was the war-time generation in the House of Lords who knew our own hands were not lily-white and feared it amounted to retrospective legislation. This week's haggling over payments to Mau Mau Emergency torture victims in 50s Kenya – reported in the Guardian – underlines that obvious truth. That too was known at the time, exposed in part by Enoch Powell, no less.

At a push you could say the same of much of the MPs' expenses scandal: systems were created and claims encouraged as automatic allowances, an alternative to politically impossible pay rises, not the out-of-pocket expenses payable only on receipt of bills, as they have since become.

The problem is judging the past – and ageing survivors of the past – by the standards of the present. So when the Harvard history prof Niall Ferguson abused John Maynard Keynes the other day he said Keynes was indifferent to the future because he was gay and childless. It was a coarse, priapic remark for which the talented-but-chippy Scot has apologised. But Keynes's free market critics promptly upped the ante. Not only did he ruin many economies, but Keynes was an antisemite who also dabbled in eugenics, they said.

Well yes. But a so were a lot of people, left and right, in the days before the Nazis made both totally unacceptable. In Dennis Sewell's book The Political Gene, the chap who first spoke of creating "lethal chambers" to deal with the imbecile poor wasn't Eichmann but the admired socialist writer George Bernard Shaw. Here are old offences, wrong but unexceptional at the time, being harnessed to prosecute contemporary disputes: austerity v expansion in this instance.

Some crimes – murders belatedly solved by DNA are an easy example – are so heinous that they always warrant pursuit. Sexual offences against women and children were played down or ignored for too long. But where to draw the line? Was Savile the Eichmann of child abuse? Hall the Barbie of taxi gropers and sexual assailants?

Should an example be made only of the big fish? How many little fish should be hauled into the net too? How do we prevent revisionist views of the recent past being harnessed to dish up rough justice to contemporary targets – the BBC, the Ukip-battered political elite or (irony) the tabloids themselves? I can't claim to know the answer, but the prosecuting authorities should be asking themselves the question.