My problem isn't that they don't trust mortals to be slave masters, that's one thing that while somewhat racist is understandable, seeing as mortals indeed lack that inclination to law, it's the fact that it sounded as though they only care about it being enslaved, because it's a genie, it's one of their bitter enemies, yet because it's very distantly related to them they would have a problem with seeing it enslaved, while they would have no problem seeing a far less vile mortal enemy be enslaved.
Their disinclination to trust mortals, to keep to the laws of how slaves are supposed to be treated is understandable, it's the fact that they only care because it's a genie that grinds my gears, if it was that they disliked seeing any being enslaved to a master, they weren't sure would keep to the laws and customs of being a 'good' master, then I could accept it, because Shaitan's are less likely to break those customs than mortals, but it's the fact that they would explicitly disapprove only because it's a genie, that really makes it bad.
From what you are saying here, I expect there's some test you have to pass to get a license to own slaves in their dominion, it makes sense they wouldn't trust mortals that haven't passed such tests to be good masters, but the fact that they only care because it's a genie makes me mad.