I would guess, that they fear, that Metastream could implement a feature, where only one of the users has paid for the content and streams it to his peers.
Those were my initial thoughts, Google misunderstanding how Metastream works. It only sends playback information, no streaming of video/audio content. Each user needs to be logged into their own session on each web service.
They have complete access to the source to verify its function. They just don't because fuck you that's why. What are you going to do about it anyway, cry on HN and Twitter?
Mozilla is Mozilla and can mount a defense so they get a license deal.