Slavery, in the Colorado and many other state constitutions, is explicitly forbidden. Colorado, at least, allows an exception for convicted criminals.
I can't believe it's 2016 and I find myself having to say "yup, slavery is bad, let's not do that."
I'm not sure of the distinction you're making, but you may be confused about categories of slavery, of which chattel slavery (what the US had until 1865) is only one. You may be thinking of indentured servitude, but even that doesn't map precisely to prison labor.
Except no, we're not going to let 100 people have those $13/hour jobs. Instead, we're going to have 100 prisoners do those jobs for free, or next to nothing.
Sorry prisoner slaves. Sorry free citizens.
EDIT: And oh, we also have to pay guards to watch them do those jobs. So not free labor, not really.
