You still need stuff from the commissary though, so unless someone is putting money in your prison account you'll need to get money somehow. I'd rather clean toilets for $5/month than some of the other alternatives (I don't know what they would be but I imagine they're not pleasant).
But if it's completely voluntary, and you won't face any direct nor indirect retaliation for refusing, then I have less of a problem with it (it still distorts the labor market when they take jobs from people not incarcerated, and the taxpayers are the ones subsidizing those jobs.)