Some of the replies here are absolutely vile: if you're going to endorse locking people in cages for years if not decades and pretend that's a justified response to anything short of their being an immediate physical danger to the people around them, then the least you can do is accommodate their most basic needs and ethical positions.
Prisons are pitched to us as places of rehabilitation - somewhere to pay penance and right wrongs before returning to the community, better for having served the time. I think it's a deeply disingenuous characterisation which serves mainly to let people avoid facing up to the reality which is prison's purposeless and ultimately harmful cruelty, but it is the dominant characterisation nonetheless.
But, if we blindly accept the rehabilitation narrative, then how exactly do we expect to rehabilitate people by fracturing them psychologically? By forcing them to violate ethical commitments which are sacrosanct to them, by alienating them from their communities and forcing them to abide by a clockwork dictatorial regime without any semblance of comfort or dignity, by leaving them to rot miserably for years?
No, and no wonder prisons are factories for broken people and recidivism if this is how people think about them. Get a hold of yourselves.
Also, before anybody retreats to the flimsy position of "but prisoners shouldn't eat better than schoolchildren" or "but what about the poor" - yes, those people are also underserved, and we have resources available to improve conditions for all of them too. All that's lacking is will.
Last but not least, if you concede that you care about neither the incarcerated nor the society they come from and will return to in time - then there's also the question of why animals should suffer? If people aren't even worthy of being afforded their basic preferences, then why should the default be the option which necessitates the lifelong suffering of sentient beings on an industrial scale?
Seriously, develop a sense of empathy.