The whole point of crawling and indexing websites to begin with is to simulate what users can and do click on. The fact that it is now technologically possible to actually track users navigation of the web doesn't change the intent of the robots.txt convention in controlling and limiting the way that search engines collect data from a site.

Bing/MS can choose to dishonor that convention, either directly or indirectly as they are now with the Bing toolbar, but I think that's a mistake on their part. Google (and Bing.com for that matter) has used the most widely accepted mechanism, robots.txt, for ensuring that content it does not believe should be indexed is not. It abides by that convention with other sites and expects other search engines to abide by that convention with respect to google.com in return.

From here the following things can happen:

1. Microsoft agrees that robots.txt should govern clickstream data from the Bing toolbar. The world returns to sanity once again.

2. The search engine community (MS, Google, etc.) agree on a new standard similar to robots.txt specifically for governing the use of clickstream data, sites are updated with new directives allowing/disallowing such use and the world returns to sanity.

3. Microsoft specifically denies that they should be limited in using clickstream data from any source, through convention or through any means. Search companies and other sites are forced to fall back on other means to achieve the same results and things get messy (for example, google blocks all users who have the Bing toolbar installed, they prevent the Bing toolbar from being installed in Chrome, MS retaliates, etc.)

For the life of me I cannot think of a sane reason why MS would choose option number 3 other than that they are insane, horribly myopic, or just plain dumb.