IIRC, you pay them (not a lot) to check out your ads and verify that they are, in fact, unobtrusive. I realize it's distasteful that money changes hands at all, but I can imagine it takes a non-trivial amount of person-hours to vet these things on a large scale, and it should be easy enough to tell if they're crooked or not just by using ABP and seeing if the permitted ads are intrusive.