Now, you could say don't do business with people who are trying to defraud you, but one of the more impressive things about the ad ecosystem is that it works without advertisers and publishers trusting each other. I can sell the space on my site and advertisers don't need to figure out how much to trust me in particular.

It's easy to have ad fraud that's plausibly deniable and maybe even not on purpose. Let's say you're a publisher and you want more people to come to your site. You look around and you find someone who says they run a newsletter and would be willing to include links to your stories for a small fee. When you multiply out the cost per visitor this looks like a pretty good deal; you say yes. This traffic turns out to be entirely bots, but you can't tell because we got rid of ad fraud detection.