While incarcerated California firefighters choose to work, many other incarcerated workers are not allowed a choice at all. The 13th Amendment ended slavery in the United States, but a loophole allows people convicted of crimes to be forced to work.
You can absolutely still have volunteer programs like that. Volunteering to work time off faster shows a willingness to work on the problems that landed you in prison. Forcing it just creates misery.
That's what pretty much all prison labor is. They coerce you with "Good Time" and something to do.
It's still slavery, it's not like they have a real choice. Do I rot in my cell or work and maybe get out a year early? Of course I'm gonna work. I'm not getting (really) paid for it, I'm just getting punished less. Sounds a lot like indentured servitude to me, which we all agreed was slavery too.