Ever since May 2008, when Google began supporting third-party cookies as part of the DoubleClick acquisition, Google has been asking AdSense publishers to update their privacy policies to include language that third-party cookies are in fact utilized. Then, more recently, Google started behavioral targeting ads and asked publishers again to update their privacy policies.
A new wave of AdSense publisher account bans has been spotted in the discussion forums and some believe this is due to some publishers not updating their privacy policies. A Google AdSense Help thread has discussion around this theory. One "Top Contributor" said:
Forum discussion at Google AdSense Help.