> But then it's not much of a jail anymore...

If you aren't free to leave, and you're kept apart from society it's a jail. No one is ever sentenced to "10 years of eating bad food". Our prison system may torture people, it may feed them maggot infested food, it may deny them healthcare or safety, but that's not justice and it's not the punishment they were given, it's just an abuse they're made to suffer because the cruel and the greedy have been able to get away with it.

If we've determined that somebody is too dangerous to live with the rest of our society there's no reason at all that they should have to be miserable or suffer needlessly. It's enough that they are kept away from us so that we're safe from them. Their actions would have required us to take their freedom, but they should be able to make the best of their situation and not be subjected to inhumane treatment or abuse.

If we feel we need to jail people temporarily as a punitive measure it's enough to keep them locked up, separated from their loved ones, and unable to do what they want or go where they want. The only people who'd think losing your freedom isn't a punishment are those who don't value freedom. Most people really do know it's a punishment, but they just want to see people suffer far beyond what their sentence calls for or the law should ever allow.