It's voluntary, (and) they get paid....

From the 13th Amendment:

Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted

"In New York, prisoners are required to “participate in programs as assigned,” including “vocational programs, industrial programs, and maintenance work assignments in any combination.” Refusal to obey an order to work is a violation of prison rules, as is union organizing and action. Both are punishable by solitary confinement and loss of good-time credits.

I cannot find a source to back this up.