Do you believe that hitting people harder will make them learn? No? Do you believe that prison will turn a bad person into a good citizen? If not, why don't you do something about it?
People like simple solutions. Out of the eyes, problem solved. Lock someone up, it will turn them into a nice fellow. Because you don't like to be locked up, you believe that others feel the same. The question remains: Why should locking up someone turn them into a better person?
Well, it does work ... for monks. They willingly lock themselves up somewhere to be able to concentrate on those things that are dear to them and where they want to improve. But there is a big difference between locking up yourself (like a hermit) and locking up someone with a bunch of criminals and guards. Just make a wild guess where most criminals learn their trade ...
The relapse rates vary a lot (by age, type of crime, prison, treatment). Still, I'm more astonished by the fact that not all of the inmates become criminal again rather than the fact that some do.
That said, it is refreshing when you meet people who don't fall for the temptation of retribution. The guys at stackoverflow.com recently discovered that some people gamed the system to hurt other members or to increase their status. Instead of punishing them, they decided to just take away any advantage of trying to game the system. Problem really solved. Carry on, commander!