Ubuntu can't include non-free codecs, but has had the ability to play Ogg Theora and Vorbis files out-of-the-box for years because they're open source. Now that WebM is such a popular, free alternative to H.264, why isn't support included by default?
Webm has to first be supported out the box in the correct gstreamer packages before it can be supported in Ubuntu out the box. The main reason it is not included as yet is because it is a rather "recent" format and code for supporting it is still a bit fresh.
You would have to wait for gstreamer to support it as part of their default set, since at the moment it's still a bit "experimental" in gstreamer.