The big networks filter such traffic, the small networks benefit from it.
https://www.reddit.com/r/legal/comments/1pq6kgp/is_it_legal_...
You may also get accidentally get your own website blacklisted or moved to a lower RPM tier, or provoke shadow-ban websites that you like to visit, or... generate more ad revenue for them.
Now if you had an AdWords account and ran a botnet that visited your property and clicked ads, that’s fraud.