Jesus. Even the US gives "enemy combatants" MREs. They are done and out of the fight. There's no reason to mistreat them at that point. It's just cruelty for the sake of it and no it's not legal.
I had to guard detainees before they were taken to the rear, and after searching them they keep their clothes and get an MRE and a bottle of water. You just sit there in silence then. Nothing else is required. The fact that they're in an armed camp handcuffed and hooded is enough to keep them quiescent. And once they're in a prison there's certainly no reason to treat them worse than we did in the field.