I don't think most people claim its rehabilitation. I think the "rehabilitation" angle is political word play and would bet that most people who support harsh prison sentences see it (and support it) as punishment.
Interesting question - why is the "political wordplay" even necessary?
"Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."
I don't see any discomfort presently, even though the constitution exempts prisoners from protection against slavery.The Thirteenth Amendment, Section 1: "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."[1]
1. Stolen from https://news.ycombinator.com/item?id=2037139
It's just slavery with extra steps.
I would argue indentured servitude is a valid way to go, but it makes people uncomfortable because they see it as slavery. Yet, keeping someone locked up in an unproductive cage, to watch them wither away is somehow "rehabilitating" them. It's not, it's punishing them, it's revenge.