As far as I recall correctly IE7 was supposed to be forced on Windows (XP) as well and was IIRC shipped with Windows Vista.
Now IE8 is forced by Windows Update
Now how is this going to change ANYTHING at all if millions and millions (okay, not that many, but you get my point) of pirated (and even not pirated) copies of Windows XP will not be updated...ever.
Remember the near uprising when windows 98 was not supported anymore. While I was "what, you still use that" thousand of people still were "Hey, I am using this...it just works...I never touched it".
Why is this different?