But in this case they are volunteers. They specifically applied to the firefighting program.
There's certainly cause for discussion about the ethics, etc. but calling it slavery or involuntary servitude is hyperbole.
Ugly giant bags of mostly water.
Another nit about Riker's argument: at one point he detaches Data's arm to demonstrate that he's a machine. Four years later Riker's arm was amputated and reattached by the subspace aliens in Schisms.
The Dominion Did Nothing Wrong^TM^
I think you're conflating the general issue of inmate labor with the particular issue of inmate firefighters.