Wow, I've actually wondered about this just to myself before. Nice post. Now we need a link on how to be a vegan bodybuilder in jail! lol. It's going to be hard to maintain mass in jail, and that is a place where you will need it most...
Actually it's easy to maintain mass in prison and build it. Lots of prisons have weightlifting equipment. Some prisons even have powerlifting teams. It's not great equipment. But it's good enough. (generally paid for by inmates) You also learn fast in prison that you don't need much equipment to work out. They use make shift stuff a lot like pullup bars, water bags, laundry bags, body weight,other cell mates (if you aren't in solitary) ect.. You can buy protein powders in the prison store. It sucks though, it's off brand disgusting stuff. But you don't really need large amounts of protein to build muscle. You need excess calories. Since most aren't very active that's not to hard to do. And there's always steroids but they are generally more expensive then street drugs so buying food from the canteen is a better use of your money.
The prisons that have actual programs like the powerlifting comps or decent equipment generally use them as a way to keep the prisoners out of trouble, because it's a privilege very highly prized by prisoners. It's good incentive to keep them out of trouble.
And that writer has a skewed view of what big chucks of meat looks like. Unless in was in a cushy federal prison for politicians he didn't get served much meat. Prisons serve cheap food, rice, beans and potatoes mostly. Very little protein is served.