I've seen that in a classical context, though I can't remember where or quite which classical state (maybe Athens? Though that might just be the "Athen was terribly enlightened about everything" trope kicking in). The argument was that the slaveowner had legal obligations with respect to his slaves and could be punished for failing them, but no-one had any obligations to a free man, and so the slaves ended up with a better standard of living. They were guaranteed a minimum amount of food, for example.