Earlier quoted context omitted.
Slave labor (which is permitted under the 13th amendment as punishment for a crime) undercuts the wages that you would pay in the market hurting both the prisoners (as they aren't paid appropriately or given appropriate medical care for injuries, etc) and the would-be non-prison firefighters. Worse, you generally can't be a firefighter once you get out of prison as they don't hire ex-convicts. Consequently you're not…
I'm not informed on the specifics on these cases, but I generally view that (many/most) prisoners have a debt to society for breaking the social contract and serving the community by making it safer is a generally positive activity. I'm happy to adjust my opinion on this topic if provided evidence that inmate firefighters either are unsafe for the community or have worse outcomes than other inmates.
It is an optional activity if I understand right.
It seems rewarding to do a training to become a firefighter and join the team, than to idle in prison.
It is a volunteer role. Many people in the world become volunteers in the firefighters, a lot are unpaid. They do it for reasons beyond money (making friends, feeling useful, helping people, etc).
A prisoner may get some carrot as a reward (like less served time) but at the end, they all benefit.