Oh, Hilary Mantel – how foolish you have been! Sure, you might have won two Booker prizes and are generally agreed to be one of the greatest writers of the modern age; one so extraordinary you have battled out of the ghetto into which female writers are almost invariably penned, with the exception of JK Rowling and, now, you. But all that pales to nothing, Hils – nothing, I say! – next to your ignorance of the politics involved in women criticising women.

Two weeks ago, Mantel gave a lecture at the British Museum, organised by the London Review of Books. Not, one might have thought, an event ripe with potential for scandal. Nonetheless, scandal issued forth most biliously (if somewhat belatedly) when someone on Fleet Street noticed that Mantel's speech had been published on the LRB website six days ago and that it contained – clutch your handkerchiefs to your mouths, readers! – comments about Kate, the Duchess of Somethingorother. These comments, incidentally, made (with Mantel's characteristic subtlety and grace) the patently obvious points that Kate's entire raison d'être now, in the eyes of the media and the royal family, is to be admired and to breed, just as Anne Boleyn's had been. She has been embraced by the royal family because she seems like the anti-Diana; one who is not interested in her own publicity but is instead depicted by the royal press machine as safe and devoted to her husband and duties.

None of these observations is new, although rarely have they been better couched. They also took up a total of four paragraphs in a 30-paragraph speech – less than one-seventh, in other words – that otherwise focused on Mantel's very perceptive observations of Henry VIII, the Queen and the nature of monarchy as a whole. But none of these can be illustrated with a photo of the eternally photogenic duchess and, more importantly, none can be souped up into some kind of non-existent squabble between two high-profile women (Boleyn being, famously if rather inconveniently, dead.) So it was Mantel's "vicious, venomous" and "withering" "rants" and "attacks" on the Duchess that made the front pages of today's Mail and Metro papers, as well as getting a prominent mention in the Independent and a somewhat smaller one in the Guardian. This kind of extrapolation is reminiscent of when a critic describes a film as "astonishingly bad" and the film poster then claims the critic described it as "astonishing!"

David Cameron has now stepped in to criticise Mantel. Photograph: Murdo Macleod

David Cameron promptly took time out of his trip in India to confirm that he thought Mantel's comments were "completely misguided and completely wrong", proving he, too, couldn't be bothered to read her entire talk before putting in his tuppence, and the result was that "Hilary Mantel" began to trend on Twitter yesterday, which really puts those silly Bookers into perspective, achievement-wise. That Mantel's speech was ultimately a call for restraint when covering the royals only adds irony to the ridiculous.

It is worth looking at what is going on here. Lazy journalism, clearly, and raging hypocrisy, obviously: what has any paper done with Kate for the past decade but use her as decorative page filler? Indeed, when the BBC covered Mantelgate (Mantelpiece?) it included lingering shots of the duchess's fair form while quoting in horror from Mantel's speech about the royal women existing to be admired. This is also a good example of how the Mail fights back when it feels it is being attacked. For if Mantel was attacking anyone in her talk, then her aim was clearly at the Mail with its obsessive, prurient fascination with Kate. To see the Mail gasping at Mantel's suggestion that the duchess is "designed to breed" when it has been on "bump watch" since she walked down the aisle is the Fleet Street reenactment of Captain Renault in Casablanca proclaiming himself to be "shocked to find gambling is going on here" while collecting his winnings. It then added helpfully that Mantel is "infertile" and "dreams of being thin". Yeah, no wonder she's jealous of our Kate, the fat childless cow.

But the liberal press has been arguably just as bad, with the Independent providing a kindly list, allowing readers to compare "the author and the princess", again emphasising Mantel's weight. The subject of women talking about women has become as fraught an issue for the left as it is for the right. The conservative press loves a good woman v woman – or "author v princess" – fight because it suggests that women are all hysterical girlies who can't be trusted with proper grownup issues because they'll start throwing tampons at one another. If, say, Martin Amis said anything vaguely similar to Mantel's comments about Kate, he would not have received anywhere near the same amount of publicity.

On the liberal side, one of the tenets of the fourth wave of feminism, which is just starting to crest, is that women should not criticise one another's life choices. Rather, every lifestyle, every fashion choice is acceptable because they all reflect a woman's freedom of choice, whether it's going out in one's underwear (Slutwalk) or, well, being a princess. This kind of open-ended tolerance is all well and good, except when it then results in people attacking another woman for expressing an opinion about an industry that exploits their own, as invariably happens when a woman discusses, say, Page 3 girls or strip clubs.

Mantel was discussing how the royal family and the media manipulate women; it is of little surprise that the media would attack her back. But this nonsense highlights how it is still, apparently, impossible to be a woman and put forth a measured opinion about one of your own without it being twisted into some kind of screed-ish, unsisterly attack. As Mantel has learnt to her cost today, it's not only royal women who are expected to stay quiet.