Maybe IT Departments would notice if google.com did the same thing aswell. Would that be monopolistic however?
I believe brand familiarity would carry a lot of people into using a different browser. Google did huge things by implementing the Firefox download links. If you go to google.com and you get prompted "Runs best with Chrome" and you get a slightly delayed service, people will switch. If they slowed the service down until its still faster than MSN, Ask and Yahoo, but slower than google's supposed to be, you'd be able to force many people over to the browser of your choosing for speed.
Also Bing doesn't work particularly well. It also complains at me because I'm using Chrome.
So you chose to switch e-mail providers rather than browsers. The parent is saying the same holds true for search engines, which have a lot smaller barriers to switching services.
(though they dont have a legit reason for blocking it - there is nothing IE6 doesnt "support" the G search needs)