It costs more than $120,000 per prisoner per year to keep them incarcerated in a local pen, and much of the cost is the security to stop them from breaking out.
Presumably they'd be getting air-drops of food, so you save up the convicts in holding cells until the next food drop, then you push them onto a Herc with the food and supplies, and give them a parachute.
When the Herc gets close to the compound it drops to a low altitude and revs up the velocity while dropping the back-hatch. When it reaches the compound it does a hard nose-up and poops out the supplies and prisoners, who will survive if they put on their chute.
It's not as expensive to do periodic flights like that as it is to maintain a full penitentiary day after day.