It seems like many or most Americans believe that the proper mandate of justice system should only include punishment because:
1) rehabilitation is immoral because it would tend to reward and promote wrong doing
2) prisoners are morally inferior and therefore rehabilitation will be ineffective
3) moral and orderly persons needn't fear the justice system because they won't break the law and therefore
4) arguments for reform must be rooted in concern for the welfare of morally inferior persons, because reform can have no other utility
I have heard these things from many people I know, and apparently most of us voted for the system we have. Also, I think we really enjoy punishing people, even if it's abstract, it's very satisfying to imagine dominating and tormenting people we don't like, and along with appeals to fear, this is an excellent way to appeal politically to citizens in order to forward one's interests.
I really can't imagine that we will stop doing this as long as only a minority of people are concerned about being imprisoned themselves.