That solves the bad bot problem, it doesn't solve the actual ad fraud problem. There are at least millions, if not tens or hundreds of millions, of ad clicks per day coming from botnets of ordinary Windows computers on residential ISPs with full JS-executing browsers. You can't detect this activity based on IP address, user agent, script execution, etc. I've seen individual advertisers targeted and get hit by hundreds of ad clicks per day, when they usually only get a dozen or two, every click from a different Comcast/FiOS/TW/RR residential IP, with unique user agents, and varying search phrases that match the ads.

It's much, much harder to detect and block that; if we could only see activity on our own individual websites it'd be nearly impossible to tell the normal clicks from the fraudulent activity.