It is a truth fast learned – a tweet can make a twit. On Friday night, Aidan Burley was tweeting about "multicultural crap"; on Saturday morning, the Tory MP belatedly grasped the popularity of Danny Boyle's Olympic opening ceremony and fired out another missive, protesting that he had been misunderstood.

The right to instantaneous self-publishing – without any of the editorial or legal checks that applied in the past – carries with it a responsibility to think what you are doing. Or, at least, it does if you don't wish to end up with egg splattered across your reputation.

Where it is necessary to clear up an ill-advised quip, the principle ought to be exactly the same in a webbed-up world as it was before: apologise a lot and don't explain too much. The old fears of looking nasty or foolish should likewise remain sufficient to police the lines of decency. Indeed, their deterrent power is redoubled by the possibility of hostile re-tweets, the modern-day rotten-fruit-throwing to which Mr Burley was subjected on Friday night.

But a high court case the same day served as a reminder that Twitter is not merely subject to social censure, but also potentially to censorship enforced by the criminal law. The court showed sanity and quashed the conviction of Paul Chambers for tweeting a jocular threat to raze South Yorkshire's Robin Hood airport, in exasperation at delays.

The remark was plainly throwaway, no more notable for its menace than for its hilarity. When some fretful soul in the airport took fright, one would have hoped a superior would have calmed them down. But nobody, shamefully, not even the Crown Prosecution Service, ever thought to step back and ask whether this fuss, whipped out of nothing, was in the public interest. Poor Mr Chambers traded his job for a conviction.

Friday's judgment said that no tweet that could reasonably be taken as a joke ought to be treated as menacing, but the Communications Act 2003 – which also criminalises "grossly offensive", "indecent" and "obscene" messaging – retains a chilling sweep.

Ever since the Race Relations Act, every legal restriction on what Britons may write with their pens or speak with their tongues has been aggressively scrutinised. Westminster has afforded necessary protection to vulnerable groups, but there are always sceptics to apply JS Mill's harm principle and insist upon sparing use of the gag. After Tony Blair got too cavalier – trying to outlaw ridicule of religion – he immediately suffered a Commons defeat.

When emails were a novelty, however, few parliamentarians paid attention to e-freedoms – unaware they had anything to do with day-to-day life. Well, now they do – and so merit the old vigilance. Tweets may invite rage or ridicule. But a tweeter's right to make a fool of themselves must be defended to the death.