It appears that inmates in modern US prisons are forced to work, though they are often incentivized to do work.[1]
Your question is still a good one though, and I would be interested to know whether penal labor would qualify as 'slavery', and whether the conditions of the facility, or the reason for incarceration would be taken into account in this judgement.
Something I don't understand about FPI is my sister in law's public school is FULL of FPI stuff although technically they can only sell to the feds. I believe there's a lot of "pass thru" and "non-surplus surplus sales" going on in the prison industrial complex. I suspect there's a lot of "Well, sorry about no federal funds cash this quarter, budget crunch and all that, but here's some nice FPI desks made by prison slave labor". The quality level of the carpentry work, both labor and materials, is usually pretty high compared to Chinese slave labor which always kinda surprises me. I wonder if folks closer to public schools than I am, see as much FPI as I do.
