Because they falsely accuse people of things, often—especially with AdSense—and that's it, you have zero recourse, your account is permanently dead. There are a ton of people who have been accused of invalid click fraud on AdSense and got banned, but who never clicked their own ads. The common Google and Google fanboy reply is to dismiss those claims and say people are lying, but I can tell you from personal experience that while maybe some are, not all are.