I don't think he'd be capable of understanding enough to disapprove in the first place.

He's much smarter than he looks, but he is a demon. And unlike myself, who has spent centuries observing and living among humans, he's not quite as... socially adjusted.

[This is a very nice way of saying he's intelligent, but also stupid.]

Ah, in any case, he listens to me. I've already told him to treat you kindly -- that alone is enough to at least imply that I care for you far more than anyone else I've met here.

[Especially since there are Consequences™ for disobeying Mephisto. They have such a beautiful, brotherly relationship, really.]