We still enjoy things even if we aren't ultimately the prime mover of our own actions.
And I think the argument for punishment as deterence falls down in a few ways:
1) Absence of punishment does not mean absence of consequences. I'm still in favor of incarceration for violent offenders to protect society, but I think that people should be treated with dignity and respect while inside. I think incarceration is deterrence enough.
2) Disenfranchisement plays a role in why people commit crimes. If they distrust or resent the system, punishment only serves to reinforce their perception that the system isn't designed to help them, so why should they help the system?
3) People need to be led by example. What example does it set when COs use violence to control or punish, or turn a blind eye to violence between inmates?
I always find it funny the way people see leadership and authority as license to put people in their place and punish those who do wrong, and then they are surprised when the subjects of said leadership aren't pleasant, peaceful people. People raise their kids to thank their parents, but then they never thank their children. They say things like "I'm not going to thank someone for doing their job" and then wonder why their kids never say "thank you". It's because they weren't leading by example, they were just barking orders.
The point is, punishment is almost always misguided, free will or no. It is a bad motivator and a bad example. The less charitable among us could still reason that "bad" people still "deserve" punishment, but realizing that we aren't ultimately in control of our own actions just underscores the point that nobody actually "deserves" anything other than understanding and compassion for being on a ride that none of us actually asked for.