We tried setting the links to rel="nofollow", we tried using a robots.txt, we even tried making the "purchased" links harder to follow... search engines crawling wishlists still took joy in marking everything on all of your public lists as purchased. Yeah, well not anymore.
The fix has the unfortunate side-effect of making those of you who insist on disabling Javascript unable to mark items as purchased. We may change this at some point in the future (perhaps requiring non-JSers to confirm?) but for now we consider this a necessary evil.