I'm not sure what the actual principle is behind this? The right to vote is a right conferred by society, not a fundamental human right. As such, it is not unreasonable for society to withdraw the right it has conferred on an individual under some circumstances.
The European Court has actually agreed that some prisoners should not be allowed to vote, but haven't been able (in 4 or 5 years?) to come up with any real ideas as to how to differentiate between prisoners who retain the right and those who lose it.
This, in my view, underpins the stance that the right to vote is only a right conferred by society which can be therefore be withdrawn. Therefore, the European Court is simply running up its own arsehole.