Earlier quoted context omitted.
You don't need to be able to prove the video is raw footage, you just need to be able to verify that a publisher you trust says they recorded it. An HMAC allows you to verify that a message (like a video file) was not modified after the author signed it.
Hypothetical: An anonymous source releases CCTV footage that appears to show a public figure urinating on a homeless person. Do you really believe that we'll all ignore that footage because the signature doesn't belong to a major news organisation?
Because the future us will be conditioned to do so by our cryptographically integrated devices. There'll be so much fake video out there that we'll get accustomed to seeing it on our devices with the big huge "fake" sign flashing on it.
(Or, again, even worse, it will not have the "real" sign flashing on it that the video from validated publishers and cameras will have.)
We can be conditioned to do this extremely quickly. Again, ActiveX is what made all the plugin security warnings start popping up everywhere. But after a while, plugins like flash and unity even fell to the reality of us having been conditioned to click "Do Not Run Plugin" whenever we saw that warning. Are we better off without flash and unity? Probably, but that's beside the point. The point is that masses can be conditioned to implement their own control when you set up a system where only data meeting a certain standard is accepted. And it's a hardware standard. Consider very carefully the position over us that such a system would put hardware makers and those with power over hardware makers in.
Now again, I'm not arguing that fakes are not a problem. They are indeed a problem. Just saying we should go about solving the problem with extreme care and caution.