It's so obvious it shouldn't need saying: all prisoners are not the same. Some are in jail because they've committed motoring offences or fraud, and they're hardly in the same league as serial killers and rapists. That's why the criminal justice system has different categories of prisons, from open prisons to high-security establishments, and I've never heard anyone argue that all inmates should be treated exactly the same. That, though, is the Prime Minister's assumption when it comes to the question of prisoner voting, which he got on his high horse about last week.

"No one should be in any doubt – prisoners are not getting the right to vote under this government," he thundered in the House of Commons. Go, Dave! Stand up for Britain and tell those ghastly eurocrats where to go! Backbench Tories hate the European Court of Human Rights, which has ruled against the UK's blanket ban on prisoner voting, as much as the European Union. They're wrong, but a weak prime minister needs to throw them a bone. He did it even though it meant humiliating the Government's senior legal advisor, the Attorney General, Dominic Grieve, who had just said that Britain's reputation would be damaged if it did not adhere to the court ruling.

Most Scandinavian governments allow prisoners to vote, while Ireland has allowed them to register for postal votes since 2006. But in this country, prisoner voting is one of those totemic issues on which the Daily Mail and Tory MPs speak with one voice. They behave as though the Government is on the verge of being forced to give the vote to the worst serial killers, even though the court has made it clear that it's up to individual governments to decide which prisoners – people serving sentences of less than two years, for example – would be able to vote. What governments can't do is maintain a blanket ban, and the reasons for that are quite compelling.

By definition, anyone who ends up in prison has broken the social contract between the state and the citizen, and one of the shortcomings of the current system is that it fails to address that problem. I'm not excusing minor offenders, but I don't think that removing a right they probably don't value very much is a reliable means of inculcating a sense of rights and responsibilities. There are staggering illiteracy rates in British prisons, and it wouldn't surprise me if many prisoners haven't ever used the vote and don't understand its significance.

Some convicted criminals have inflicted such terrible damage on other people that they should forfeit some of their civil rights, but I'd like to see the rest encouraged to think of themselves as part of society rather than outside it. One minute Cameron says he's keen on rehabilitation to cut re-offending rates, the next he's sucking up to his party's europhobes. Don't expect consistency or principle from an embattled prime minister who's spotted an opportunity to pick a fight with the hated judges of Strasbourg.

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