> Being able to force someone to do anything against their will, minus the right to sell people, still sounds a lot like slavery to me.

It sounds like you have your definitions messed up. People being property is the defining characteristic of slavery. That would almost always entail involuntary servitude, but involuntary servitude is a more general term that encompasses other practices.

Also "being able to force someone to do anything against their will" certainly doesn't apply to prisoners in the US any other developed country.