I disallowed MSN bot via robots.txt many years ago. Shortly after Bing started up, I started getting hits coming from Bing. I checked their forums to see how to disable Bing from crawling my site, the instructions hadn't changed - disallow MSN bot. Each time MSN bot came along, it got robots.txt, then apparently went away (that IP address didn't repeat).
Since I couldn't stop Microsoft from linking to my site, I got my revenge a different way. I wrote a filter to check the referrer; if it came from Bing, I redirected to Google with the same search parameters. Most come back from Google a minute or two later (after their confusion wears off?).
I don't care so much about the legality of what Microsoft is doing. It's just plain wrong in my book, and I'll happily lose potential hits to my site to see to it that they do not benefit from this underhanded behavior.