Except that they can't without using proxies on the outside. Instead, we decide that they no longer have a right to communicate or express themselves to anyone outside the prison.

It seems like a more effective and humane method is to allow them free telephone usage, with the complete disclosure and knowledge that their conversations are being listened to/recorded. If crimes being driven by their command are executed on the outside, it can be found out how they communicated that command, and the people responsible for committing that outside crime also punished.