So I think enslaves is a bit much here.
This is a voluntary program, and I'm not getting the impression it's the do this or we punish you voluntary either.
Meanwhile there are actual prisons that will force labor or severely punish you for refusing. That's enslaved.
It's also possible that the fire fighting is voluntary, but if not this, it would have been other mandatory slave work.
Yes there are slaves in prison, I'm just not entirely sure this specific circumstance is it. They are getting slaves wages though.