AFAIK it's voluntary, no? Also, given the choice between working 8(?) hours a day, or sitting staring at a cell wall for the same hours, which would you pick? (I'd pick the work)

I'd absolutely choose staring at walls to cleaning toilets for $5 a month.

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.)

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).

It's voluntary in that you can choose to work or you can choose to be thrown in solitary, beaten, and starved, until you stop refusing to work.

Voluntary in the same way every mugging is voluntary.

Being in a work crew is optional. You can sit in your cell all day.

Anything could be argued. Doesn't mean there's any actual basis for the argument.