Because worrs carry not only direct meanings, but subjective connotations, and most people consider slavery an unjust subjugation of another human being, and think it is immoral by definition, in any circumstances. On the other hand, even most of the people who aren't fans of US prison system still consider the general idea of prison to be just, as the general idea of prison labour as a way to repay society.
Idk how this works in the US in particular - but I suppose that - essentially - yes. Harsher conditions.
When one has a essentially complete control over another person's life, there are ways to make this life hell, even while staying within the legal bounds.
From my understanding, the main punishment for not working as prison labor is losing the small wages you do get (i.e, normally one of the punishments for misbehavior is prohibiting you from working).
