Has the European Court of Human Rights breached its mandate as a court on Prisoner's votes?

This is a classic example of matters of national polity being decided by international courts and institutions using open language in treaties to their advantage. Surely this is something that should go to elected domestic legislatures for a vote? There is nothing in the European Convention of Human Rights that could have made this a predictable for States entering into the treaty. This surely renders the courts judgment illegitimate according to treaty law. (See Article 31 Vienna Convention on the Law of Treaties, which states that only literal interpretations are permitted I.E. Non ideological ones).

http://www.bbc.co.uk/news/uk-11671164