Presumably they would do this because there would be a set of high-quality, powerful web design tools that ran better on Windows than on the Mac. If this were the case, it would make the most sense for these hypothetical design tools, regardless of who makes them, to work closely with Microsoft technologies like Silverlight when it comes to RIA development. No other company would be able to develop an RIA framework that beat out Silverlight because Microsoft would have the home field advantage of building their framework on top of their own operating system.

This is a thought experiment, and not reality, only because there is no set of good design tools that run on a Windows PC. Enter Photoshop Express, Splashup, and Picnik. They run in a browser, so by definition they run the same on both a Windows PC and a Mac.

Interesting.