I was always under the impression that robots.txt was intended not to control search results, but to help prevent overzealous crawlers from taking down your website.

If you want to control what a search engine does with a page, you should use <meta name="ROBOTS"> and rel=nofollow, neither of which Google includes in its results pages. It would be reasonable for Bing to look for and honor these directives when collecting their clickstream data, and Google isn't availing itself of the option.