If extensions checked to see if they had permission to alter/interfere/access webpages by looking it up in robots.txt, which is what is being suggested by people who say "Bing toolbar violated robots.txt", then that same mechanism could be used to specify:
User-Agent:adblock
Disallow: /
Or even, if the "User-Agent: (star)" is supposed to tell Bing Toolbar not to play about with the page, then why doesn't that also apply to all your other extensions/addon/BHO's/plugins whatever you want to call them?
I think that's what most people though was wrong in your comment.
Care to elaborate on how an adblock extension could be excluded via robots.txt in that fictive scenario?