Fake Website
Fraudster set up fake sites that are made only to serve ads to bots. It is usually a three step process whereby a fake website is created as a first step. As a second step, cheap bot traffic is purchased and routed to the new website. The ad networks see that this site is getting a lot of traffic and include it in their inventory. The third and final step, advertisers buy ad space on the site and the fraudster gets paid. Ad fraud is that easy.