That would certainly have been a valid reason. But the actual reason is irrelevant to my point: that some military prisoners have a tangible value that justifies the (relatively minuscule) cost of their incarceration, whereas common criminals (whose incarceration make up 99.9% of prison budgets) have no real value to society.
[Which, BTW, is why I have long advocated that the penal system should require inmates to work and pay for their incarceration costs, in addition to prosecution costs, law enforcement costs, and victim restitution.]