Wow you're really reaching here now, aren't you? The bitstream format and software are licensed under a Creative Commons Attribution License and the BSD license, respectively.

By your definition anything not in the public domain is proprietary.

Let's get another definition. This one is from wikipedia: Proprietary software is computer software licensed under exclusive legal right of the copyright holder. The licensee is given the right to use the software under certain conditions, while restricted from other uses, such as modification, further distribution, or reverse engineering.

As far as I am aware of, any user of WebM are NOT restricted from modification, further distribution, or reverse engineering. The beauty of the current licencing is that it can't be revoked; it is already free. Google can develop future versions and lock it down (suicidal move, though), but as it is the community has its hands on it.

So to be clear, you admit that WebM isn't proprietary, but future versions could be? That's true of every open source project on the planet, that's how copyright law works.