Google finds itself in the middle of quite a few sketchy markets where they both profit from the existence of the markets, yet potentially could also benefit from destroying the markets.
The current example is domain kiting where domain registrars temporarily register (yet avoid paying for) massive quantities of domains, throw up some Google AdSense ads, then collect up to 7 digit checks monthly from Google (after Google's allotment).
This type of advertising provides no value to customers while generating millions and millions per month for Google. Google has decided to destroy this market by banning AdSense ads from domains registered for less than 5 days. That's a simple move that has little effect on legitimate publishers.
It's a tricky situation since advertisers, who's ads get clicked on generating charges, may or may not have a problem with this. If the ads convert at a reasonable rate, no big deal. But is it really a market worth enabling?
An ongoing example that gets me more worked up is splogs. Why is it so easy for people to set up thousands of junk blogs on blogger, throw AdSense ads on the blogs, ping the crap out of blogs across the web, and actually make money?
This seems like such an easy thing for Google to detect since it generally uses at least two Google services: blogger and AdSense. Shouldn't they be able to do a better job detecting splogs and suspending the AdSense accounts of sploggers? Or, at least driving down the value of clicks from AdSense ads on splogs to a low enough point that splogs are no longer economically feasible?