I'm not sure who would classify prisoners as free, though. Classifying prisoners as free is warping what most folks would consider free. You can have rights and not be free. POW's are supposed to have certain rights, too, but that's hardly free. You can have rights without being free. Not being chattal slaves doesn't make you free either, nor does it mean you aren't a temporary slave.

In general, folks would agree that if you are in a situation where you are forced to eat at certain times, possibly only use toilets or showers at certain times, can be used as labor with little to no pay, and can never roam freely at the behest of armed guards that can shoot you, you aren't free. The main exception to some of this is military, but for now, this is something folks choose and some countries' soldiers are more free than the ones in the states.