The automatic validation of sites that display network ads ensures that the sites in the ad network aren't banned by google. (No pages in google means no ad weight) However, there is no equivilent validation for the urls pointed to by the ads we display. It's possible to use the ad weight from an unbanned site to create ads that point to a banned site. The current human validation method should catch most of the bad sites when an ad is created, but will not necessarily catch a site that alters it's content after the ads are approved. If ads are required to undergo a periodic automated review to determine if the site the ad points to has been banned by google it may help weed out undesirable sites. This would be in addition to the existing human review system.