[why don’t they grow their own?]
Because expecting them to WORK will be seen by them and SJWs as cruel and unusual punishment.
Plus, you might have some demographics in the prison population that might object to working in a field....
I think post #15 has it closer to the truth.
Growing vegetables is not only a useful activity, but it is very therapeutic. All the evidence suggests that the jailbirds who engage in useful work such as farming and ranching are much less likely to return.
There are cases where they are engaged in other useful activities such as breaking wild horses, training seeing eye dogs for the blind and even common industrial work such as making license plates give positive results over the status quo warehousing and coddling model.
In Japan, all jailbirds are expected to work 10 to 14 hour days with one day off. Those doing more strenuous work get the 10 hours scaled to those doing the least getting the 14 hours. Prison rape and other forms of violence is near zero. Recidivism is typically less than 20%, compared to nearly 70% with our jailbirds.
Interestingly, a good share of that 20% are simply people who've grown accustomed to prison life and want back in. They typically get back in by committing some fairly non violent crime such as riding a train or hailing a taxi with no money to pay.
There is even a television series about some of the creative ways these guys get back in prison. One of these guys enjoyed seasonal work as a farm hand, but by the time late October rolled around, this work got scarce and the temperatures got cold, he had a friend who was a taxi driver who'd drive him back to the prison and not pay. The prison staff was in on the gig and would check him in and, a few days later, call in a judge who would sentence him to prison until the following spring when, of course, he'd get out and go back to work as a farm hand.
This time, of course, his friend the taxi driver would be waiting and get paid by the prison to drive him home, only to repeat the cycle the following autumn.