Google charges ads per click, possibly mainly for that. Also it's a preferred solution to asynchronously ping the log on click rather than a synchronous redirect hack
i'd guess the mechanisms for ad clicks and search engine refinement are largely separate (with maybe some overlap for sponsored results? does google do that?).
the asynchronous ping attribute is new[1]. i'm pretty sure it was the mid-00s when i first noticed that search result links bounced through a redirect via google. (and i'm guessing it was added to reduce confusion when hovering and ameliorate copy/paste issues for search result links, but i don't know for sure)
