Putting criminals to prison in itself is rather questionable from a certain point of view... Its also the only way to keep certain loonies away from causing even more havoc. If humanity would be even a tiny bit more sane and nice, I would be against the whole system. Bit like armies too.
Keeping people in after they have finished their sentence is just criminal in itself. Unless the convict is truly dangerous and WILL commit his/her crime again straight away after getting out, but who can tell that? Who's so almighty to know in advance who's gonna do what? (Roll in the thought police here.)
The whole "criminal justice" system is quite fucked up too, people go to jail for ridiculous things and the real criminals are molesting the whole world with impunity and are protected by law or just plain above it. Calling this place a prison planet may not be too far fetched at all.
And how about those who just keep getting back in the stone house again and again? I read some article, that most of those who do not end up in prison again have been there only once, more time in prison = greater chance of further crimes and alienation from the society. So who's fault is that, is it the criminal's, or are those crimes more like a symptom of the system malfunctioning?