They're not being compelled to be firemen.

Well, under a very narrow definition of "compelled," maybe not.

Are you implying one could be convicted and sentenced to be a slave ?

No. I am implying they weren't sentenced to be a firemen or a slave so they shouldn't be. People have been sentenced to do community service which I generally don't think is compensated. Going to be honest, I am ignorant and haven't been incarcerated so I am lucky, but from what I can tell community service tends to be a lighter punishment than prison, and doesn't tend to be as exploitative as the uncompensated labor described by these activists. It is also proscribed by the court and not up to wardens and their friends to decide its terms and extent.

And no, I don't think anyone should be sentenced to be a slave or a firemen. Any such sentence probably would violate the 8th amendment.

[0] Insightful: https://news.ycombinator.com/item?id=17813698

Sorry, chalk it up to fatigue and the consequent decline of my comprehension skills. I don't know why I misunderstand that sentence so badly.

It used to happen in a quite literal sense: https://en.wikipedia.org/wiki/Convict_lease

The modern prison labor system varies from this in degree, but not really in kind. Inmates are still compelled to work in the US.