An anonymous individual posted a short interesting video on search "arbitrage" aka PPC search spam.
Well worth watching.
The interesting thing here is that Google tracks the original AdWords click onto the spam page, as well as the subsequent AdSense click off the spam page. And Google has a cookie on the user's system.
Doesn't seem that technically hard to disallow AdSense payments on clicks following a Adwords click, within some short time frame, same browser, on related words...
If Google wanted to stop this practice, seems they could stop it dead in its tracks with an engineering fix.
What say you, Google?