Compare to Microsoft's plugins for Chrome[1] and Firefox[2], which have to actually override the <video> tags with WMP embeds because Google and Mozilla don't provide any mechanism for the user to extend codec support.
Microsoft doesn't provide any mechanism for the user to extend codec support (with the notable exception of WebM) either. Try to add WMV or Theora to IE9's HTML video support and see what happens. Their original announcement said no extra codecs would be supported, but they later made a special allowance for WebM. I'm interested in the technical details of how that actually happens since apparently it involved Google engineers being on site at Microsoft to get it working rather than just plugging into an existing extension point.
This seems more like a way for Microsoft to avoid (perceived) patent liability than a technical decision about the best way to add codecs to a browser.
This seems more like a way for Microsoft to avoid (perceived) patent liability than a technical decision about the best way to add codecs to a browser.