Tumblr's content got smacked because of incompetence and laziness. They had a bad CP problem, like worse than masterchan probably. Instead of trying a more specifically crafted attack towards the CP posters, they just decided to do a blanket ban.
In reality, it hasn't anything to do with advertisers, the tumblr staff is just a group of lazy incompetents (See: the flagging of the post that announced the new spam filter by tumblr staff). I mean, come on.