Megan is not responsible to tag him in her post, because that IS an advertisement. If she wants to, then so be it. Many artist may have it negotiated to promote their team, maybe for a discounted price. Just because you're rich, doesn't mean you can be smart and look for discounts.
I think the difference here, is lying, Megan is clearly lying which is damaging. This could damage his reputation, if he is saying that he does her make up, but she is going around saying she does it herself. I saw another interview where she did the same thing, saying which eye shadowed, and foundation she had put on that day. She made it seem as though she does all of her makeup herself.