Bing didn't crawl Google's pages (so never reads a robots.txt file), they are collecting click stream data from users via the Bing toolbar as they browse a page

Bing didn't crawl Google's pages (so never reads a robots.txt file), they are collecting click stream data from users via the Bing toolbar as they browse a page. It's voluntary, you can opt out of the anonymous data reporting. The data they are using belongs to the user, so it's the user that can opt out, not Google.