Yeah, no kidding. It's the perfect activity for a bunch of guys trapped in a fairly unpleasant place. It eats up *tons* of time, keeps your mind occupied, and provides a dose of escapism. Plus it's a lot of fun as long as you're not playing with a rules Nazi.
The problem is that most prisons forbid dice due to gambling concerns. A guy I know who ran D&D games when he was in used a system involving random ticks of a rubber pencil on a piece of paper produced at the beginning of the game, and I couldn’t really figure out how that worked by his description.
PC's raid the nearest goblin dungeon, DM railroads the party through a series of DC 25 traps and they all end up in Goblin Boss's gaol. Campaign begins.
The humans in the party segregate themselves by race and class, and one of them tries to justify the Drow system of slavery utilizing arguments about the inherent value of human labor.
The hook is all the screws are addicted to cave fungus, and they need to find a way to get them clean, whilst navigating lifers peddling it to anyone they need to control.