Umm... Adsense doesn't need a referrer and it definitely doesn't need search terms. Adsense crawls every page that it serves ads for and that's going to be better context then any analytics packages would offer.
What someone searched for is very valuable when it comes to context, you can crawl and index a page to see its about iPod Accessories - but you can look at the referral and see the person searched for iPod Touch Case and make the ads that much better.
Adsense uses behavioral targeting, of which a part is based on the search queries you enter into Google (as in, when you're searching for "christmas toys" on Google, for a certain amount of time you become a hot target for relevant ads in Adsense).
Other ad providers were able to mimic this by analyzing the referrer urls from which people were being redirected. This is not possible anymore. If Adsense doesn't disable this functionality, then Google is indeed acting anticompetitive.
Apparently you're correct, I have looked up the privacy policy and was unable to find any indication they're using the keywords you're searching for for Adsense.
