The punishment for theft historically was (1) execution, (2) then whipping, the stockades, or some other physical abuse, and (3) more recently, imprisonment.
Consequently, in the historical scheme of things, imprisonment is considered humane under the Constitution.
More recent SCOTUS decisions are starting to swing toward more humane imprisonment terms, but it may be a long time before SCOTUS actually rules on this issue. Progress will come from the state and local levels.
You know, whipping and stockades might have been more immediately abusive, or cynically speaking, more visible to the outside world, but at least they were brief. I wouldn't be surprised if it turns out the psychological and physical harm of long-term incarceration was actually worse. (It might be more effective, on account of the criminals being physically restricted from committing crimes in the outside world, but it might also be more damaging.)
More recent SCOTUS decisions are starting to swing toward more humane imprisonment terms, but it may be a long time before SCOTUS actually rules on this issue. Progress will come from the state and local levels.