The browsers that do support it run on the platform by the same vendor.
I don't understand the significance. The same APIs are accessible to any other browser. If someone has evidence that Mozilla, et al. have been technically locked out of using the media APIs on Windows or Mac OS X, I'd be interested in the details, and if someone has evidence that they've been locked out of using the media APIs on Linux, I'll eat my hat.
they use their vendor's decoder and nothing else
I'm not sure what you mean. If you mean something like "Windows will only support h.264 and you can't use anything else", then that is exactly wrong.