The surprising thing about the case of Dominique Strauss-Kahn and the alleged sexual assault of a hotel chambermaid isn't that all charges have been dropped. It's that he was arrested and charged in the first place, given how unlikely it was that he would be convicted. Even if DSK hadn't been one of the world's most powerful, well-known men, the chances of his being found guilty and going to prison were always low, as they are for most men who find themselves accused of rape or sexual assault.

"Rapists who end up being convicted in a court of law must regard themselves as exceptionally unlucky," Professor Joanna Bourke of Birkbeck College argued in her magisterial book Rape: A History from 1860 to the Present. "Rape in this country [the US] is surprisingly easy to get away with," a special report for CBS News concluded in 2009.

Nationwide figures for the US are hard to track down but an analysis of Department of Justice statistics by two of the country's leading sexual assault experts in 2009 found that conviction rates hadn't improved since the 1970s; their study suggested that only two per cent of rapes reported to police in the US ended in a defendant being sent to jail. Across a number of countries, a rising number of reported rapes has not led to a corresponding increase in convictions.

There is no mystery about this. The hunt for the ideal rape victim is never-ending but fruitless, for the simple reason that it requires unimpeachable conduct on the part of the victim in every area of her life, past and present. Women who have been drinking, who know their alleged attacker or who've ever told a lie to a public official, even in an unrelated matter, are not victims prosecutors want to put before juries. Bourke makes a similar point in her book: "Jurors, defence counsel and judges not only expect a much higher level of resistance than required by law, they also require a greater degree of consistency in rape testimonies than they require from victims of other violent crimes."

Indeed what's fascinating about the case, which comes down to the word of the former head of the International Monetary Fund (IMF) against that of an immigrant from West Africa, is that in the beginning prosecutors clearly believed her story about a violent sexual assault. Strauss-Kahn's semen on Nafissatou Diallo's uniform and the carpet of his suite proved beyond doubt that a sexual encounter had taken place, while medical evidence and Diallo's evident distress appeared to support her claim that it wasn't consensual.

So confident was the Manhattan district attorney, Cyrus Vance, that he proceeded to charge Strauss-Kahn, turning him into one of the world's most high-profile defendants. Even on Monday, when Vance asked a judge to dismiss the charges, he used an ambiguous formulation to explain his change of heart: "The nature and number of the complainant's falsehoods leave us unable to credit her version of events beyond a reasonable doubt, whatever the truth may be about the encounter between the complainant and the defendant."

This is an absolutely classic outcome, signalling not the vindication of the defendant but the prosecution's judgement that the accuser would not make a good witness. "Dismissal does not mean he is innocent, simply that the district attorney doesn't believe the case can go to trial," observed a French lawyer, Pierre Hourcade.

Vance's own words suggest that his decision was based not on Diallo's account of the alleged assault, which contains only minor discrepancies about her behaviour immediately after her encounter with DSK, but lies she told when she arrived from Guinea and claimed asylum in the US. Such behaviour is not uncommon when would-be immigrants are trying to improve their chances of being allowed to stay; Ayaan Hirsi Ali, the Somali-born Dutch MP, resigned from parliament five years ago after admitting that she lied on her application for asylum in the Netherlands. Ali later moved to the US where her views are treated with respect and she is regarded as a trenchant critic of radical Islam.

If Diallo's persuasive account of a violent sexual assault is to be dismissed because she lied to get into the US, the implications for other immigrants are alarming. Are prosecutors really saying that anyone who has lied on an asylum application cannot be considered a credible witness in an unrelated matter, no matter how many years later and regardless of forensic evidence supporting their claims? This is surely setting the bar too high, as well as sending a message that some potential victims cannot expect the protection of the law. It's well known that sexual predators often choose their victims with care, selecting girls and women who are "vulnerable" in some way – black, poor, working-class – and likely to make less convincing witnesses.

In the case of DSK, there is also a nagging question of double standards. Some of his more excitable supporters in France have already floated the idea that he could resume pursuit of his ambition to become the Socialist party's candidate in the presidential election, as though he has emerged from this affair with a spotless reputation. But fairness demands that his past conduct should also be examined with equally rigorous standards, and the picture that's emerged is far from edifying.