This isn't support in the sense that Microsoft will ship this codec, just if you install it you'll be able to use it.
For me "it's up to the user", is not really supporting it any more than Microsoft supports any other third-party software. I imagine that Microsoft's implementation allows any codec installed to be used in html5.
Supporting VP8 and allowing VP8 content to play if the user installs a VP8 codec are two very different things.
Microsoft whitelists all codec but h264 on IE9, so allowing VP8 is a sign of support, not very strong but quite a bit. Also remember that first versions of windows XP had a similar kind of support with mp3 until it started shipping with it once it was super popular IIRC.