Indeed. Certainly it is technologically possible for clickstream based indexing to still abide by robots.txt rules. However, the Bing toolbar does not. That is the key issue here.
How do you know it does not? I'm assuming robots.txt is about preventing the page contents from being crawled and added to the index. If all they use the click info for is to associate referrers (google URLs in this case) to pages in the index and they don't crawl the google search itself I don't see how that breaks the robots.txt contract.
The page contents are being crawled and added to the index, but by Bing Toolbar users, not a computer program. I consider that to be an underhanded way to circumvent robots.txt, but others might not.
What makes you say that? We haven't seen anything to indicate Google search pages are in the Bing index.