Your uncle is correct in that work isn’t given to inmates for their benefit or edification. It’s to line the pockets of those in charge. If the state wanted prisoners to more easily ‘re-integrate’ into society, they would let them form unions and pay them a decent wage for their labor. Instead, they’ll either abuse their authority to get the inmates to work for peanuts and pocket the difference, or they’ll pass the work to one of their cronies.

One of the roles of prison labor is to undermine organized labor outside of the prison. Prisoners are often employed outside of the prison in addition to doing work to maintain the prison. By forcing them to compete for jobs with slaves, it undermines non-incarcerated laborers’ bargaining power and pressures them to accept conditions closer to incarceration.

I don’t blame your uncle for his position; life in prison is extremely tedious, and the quality of what you’re given makes commissary essential. People outside need to organize with people inside to create a situation that benefits both.