Why Microsoft cares
Despite all the purring they have been doing recently regarding their new found love of open standards, IE *still* represents a haven of proprietary protocols that MS can use to maintain their user lockin on the underlying OS. There are plenty of browser based apps and websites that simply won't work in any browser other than IE and while that might be slowly changing, Microsoft are now attempting to inject a new layer of lockin to the web via silverlight.
Ultimately, they won't really care what browser people use, as long as some part of the web "experience" relies on MS proprietary tech. If Silverlight ever achieve the ubiquity that MS hopes for then they can, to a degree, take less interest in browser market share and rely on the fact that users require Windows to run Silverlight properly thereby protecting their OS monopoly into the future.
Fortunately, Silverlight has been an epic fail up to this point so they are forced to continue trying to protect their browser market share while at the same time trying to look like they care about interoperability and open standards.
Evil Bill because THERE IS NO EVIL BALLMER ICON!