The implication is that Microsoft realizes that some people, believe it or not, might want to use VP8 in a context other than web video, and so the proper place for the codec is installed as part of the system, where it can be used by all video playback software, instead of built into the browser where it is only useful for web video.
If that's what they want to do, they could still have the IE9 installer install the codec system-wide, but it doesn't sound like that's what they intend to do.
