One of my employees has a very honest and decent content site (not MFA-style). He ran AdSense on there for a time before they shut down his account and ignored the appeal. He suspected a friend of his clicked a few ads to help him out, without instruction or without letting him know. Completely out of his hands. It would be trivial for people to do this to their competitors.
I suspect that sites generating more income for Google can get away with a lot. The smallfry just aren't worth the hassle to investigate.