The whole idea of a franchise is so people can get the same menu at the same quality at the same price no matter where they are. Having dynamic pricing means you can't even be assured of the price at a single location, let alone being in a new city with an unfamiliar restaurant.
If you don't know what you are going to be able to buy with your $5, then might as well go to someplace different.
Maybe the guards just administer some additional unprovoked beatings to the inmate to ensure they aren't in a physical state to parkour over the wall