In the US, the purpose of incarceration is cheap slave labor, which is why oligarchs like it this way. Can't have that if inmates have rights and are allowed to have non-slave jobs.

Prison workers in the US are generally exempt from workers' rights and occupational safety protections, including when seriously injured or killed.[43][39] Often times, inmates that are often overworked through penal labor do not receive any proper education or opportunities of "rehabilitation" to maximize profits off the cheap labor produced.[44] Many incarcerated workers also struggle to purchase basic necessities as prices of goods continue to soar, meanwhile prison wages continue to stay the same.