I don't think that that's a good idea, because it implies trust when there actually isn't any.

Being signed with something just means that whoever has that key could've done that. That might be the owner of a specific camera, but it might also be the camera manufacturer, anyone else in the supply chain, or anyone who dumped the key.

Imagine fake evidence signed with the same key as your camera uses being used in court against you. And the court believes it because it has this signature attached and those computers are very secure and all.

Exactly that will happen. Not widespread, of course, but it will.