This is either funny, crazy, hypocritical or just good politics depending on your point of view.
You could argue that continuing to deny all prisoners voting rights is a great bit of justice and will bring home to criminals the perils of breaking the law. Unfortunately, the policy has been ruled unlawful by Europe and continuing to apply it is itself illegal. Which sends a rather curious message to offenders.
On the other hand, the policy is said to be enormously popular with voters - and "standing up to Europe" even more so.
There's also a crazy idea that society should attempt to rehabilitate offenders. Allowing all of them a (postal) vote would be a cheap and easy way of sending a message that we're serious about it.