Once something is unencumbered, there is nothing to stop forking. For example, Mozilla could take VP8, use a new container, evolve the codec, etc. There's the difference.

You try and fork H.264 you get sued.

The fundamental freedom in "open" is freedom to fork. Everyone forgets that.

VP8 and WebM are formats, not standards. Interesting read: https://en.wikipedia.org/wiki/WebM