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.
Agreed, what about communities where marriage is extremely strict or are arranged?
-they essentially cannot leave
-they must work or can be beaten
-they cannot go out alone
-money is often exchanged in the marriage process
This meets all the listed criteria for slavery. No black and white (pun not intended, but it works) definition that can be used, so any quantification is subjective.
On the subject of prison labor, just after the civil war many southern states used trumped up charges to essentially enslave the newly "free."
People also do a good job of rationalizing slavery into something else. The Swiss verdingkinder (child contracting) program existed until into the 1970's and some people still don't believe it was child slavery: http://www.nytimes.com/2014/11/11/opinion/slaverys-shadow-on...
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.
[1] http://en.wikipedia.org/wiki/Penal_labour#United_States