Yes, it's unfair to force a human being into labor. Paid or unpaid. It should be voluntary. Community Service as a punishment is different. That's a do labor or else do time so you're trading labor for hours/time. Paid in trust. I have issue with do time and do labor as a form of punishment without reward.

Most inmates are incarcerated due to circumstance. They lacked the ability to better their lives and required monies to live, so they resorted to crime. Given a chance, many inmates get degrees behind bars, learn skills, write books, practice art.