The whole concept is so bizarre to English eyes.
At a serious level they were found guilty of a crime so should serve a punishment but more light-heartedly what life - fed every day while you rebuild muscle cars.
If you study US history just after the Revolutionary War, one thing of note would be the significant changes made in the legal system versus that of the British. These changes included jury trials, presumption of innocence, and prohibition against cruel forms of punishment. A great effort was also made within the US justice system to rehabilitate criminals rather than simply punishing them. In his classic 1835 work, "Democracy in America", Alexis de Tocqueville writes quite a bit about this.
Giving prisoners something to do that makes them feel productive is the best way to keep them from becoming disruptive. And working as an auto paint/body man is one of the few jobs available to someone with a felony record that pays enough money to survive on.