> and not straight up forced labour?
Well you’re making the assumption that prisoners are forced to do this work rather than opting to in order to make a little money for snacks and/or make a case for good behavior when they come before the parole board.
It's still slave labor if the most basic of comforts and privileges are locked away from you if you don't participate.
Plus you don't really have choice in the labor you perform, no choice in where you perform it, no choice in when, you aren't really paid, you can only spend money in the commissary (at insanely inflated prices).
Sure it's not a slave on a cotton field getting whipped for not meeting quota, but it really isn't far from that.
> make a case for good behavior when they come before the parole board
It can be a bit more explicit than that: in Colorado, inmates can earn 10–12 days per month of "earned time." Earned time shortens the time until eligibility for parole. Section D in the linked document (from the linked department policies page section 625-02) gives examples of behavior that can add up to earned time. For instance, a day of work at a disaster site is worth a day of earned time (D.4.a.1)
https://drive.google.com/file/d/1q6IXf-yWnbA3Ujjejola7fiwijv...
https://cdoc.colorado.gov/about/department-policies