That's the first thing that should come to mind since WebM includes Google's own competing codec, VP8. The project was announced last May. A codec, for those unaware, is used to encode and decode video and audio to be played through a computer or other device.
It's also strange that Google would declare a move to openness when it's made no move to boot another proprietary standard, Adobe's Flash, which uses H.264 and other codecs internally. As pointed out, however, Adobe is one of Google's partners in WebM, so there's the obvious reason.
The H.264 standard has been around since 2003, and is indeed proprietary. Still, with so much video using it on the Web, this seems a move that will shaft users of Chrome. It is true, however that Google is a corporation. As such, the needs of shareholders outweigh the needs of the end users.
In fact, one commenter at the Google post, shidoshi, put it quite well:
Ugh. This is a move by Google where they care more about the open source "community" than they do actual users of their browser. Let's be real here: WebM has a LONG way to go before it will have any serious amount of traction, and Theora is a joke. Like it or now (sic), h264 IS becoming the standard, and dropping support for it for no good reason is ridiculous.As we said before, there is still Flash support, and somebody will likely write a plug-in to replace the built-in support. However, it appears the Web "video wars" are alive and well, HTML5 or not, because although the HTML 5 standard adds built-in support for video, it has not yet specified a codec.
Current, in eyeshot, I have six devices that can all play digital video: a PS3, an Xbox 360, a laptop, an iMac, a PSP, and an iPhone. Guess what one codec each and every one of those devices is able to play? h264.
If I want the widest audience possible to be able to access my content, why in the world would I encode it in either WebM or Theora? I wouldn't.