I had a flash of insight (aha!) this afternoon thinking about other media where fraud is perpetrated. It's possible to commit competitor phone fraud by frequent dialing of their 1-800 number in the US. It's also possible for an interested party to boost the value of a telco's stock by frequent dialing of 1-800 numbers. (Both of these assume that charges are made as a function of the number of calls.) I checked Wikipedia and it turns out these types of fraud are documented. In addition, it's possible to commit a type of "publisher" fraud (analogous to clicking on the ads appearing on one's AdSense site) using 1-900 numbers. I actually wasn't aware of this; I didn't realize that some of these services were funded by a cut of the fees going to the service provider.
Again, when AdWords and AdSense were launched, why didn't Google think of this?