It's a cat and mouse game. Google gets good at it; scammers get more sophisticated; google catches up again; ad infinitum. Also, in addition to the scam mentioned above -- in Google's case, targeting AdWords -- they have to deal with scammers hitting their own AdSense properties.

It's a well known problem. Considering AdWords is Google's primary source of revenue, they work hard at it. For a tiny bit more directly from Google, see: http://support.google.com/adwords/bin/answer.py?hl=en&an...

Overall it's hard to say whether google wins or loses overall by this. In absence of a real competitor in the search ad space, people generally will swallow google's fee hikes ...