I understand this, but it's suspicious when they manage to detect the fraud only when it's time to pay out the revenue to the site owner. It's also a little unfair to assume every detection of fraud is 100% purposeful action by the site owner and permanently terminating their account without even so much as a warning. In my case, I had ads running on my blog. It took 4 months to rack up $100. I think the chances I was purposefully defrauding them was low, given how long it took to get to the payout point.