I think turning off body cameras without permission should mean an automatic dismissal from the police force.
If they actually turn it off, I agree.
The problem is twofold:
1) Current power supplies are too big to support recording for a whole shift and then some. In practice cops turn on their cameras at an incident. Mistakes will be made. I believe this could be addressed but it would require new cameras. Put transmitters (say, bluetooth) in police cars that tell the camera to shut up. Lose the signal, the camera comes on--ideally, make it come on 20 seconds before it loses the signal. (No, you don't need time travel. You can actually run the camera all day with current batteries, the limit is in storing it. Leave the camera running and encoding the video but throw it away when you run out of memory. When you turn on the camera that encoded video is written out. This is nothing novel, that's how parked incident recording in dashcams works.)
2) There are times cameras should be turned off. Think a cop is going to be welcome in a public bathroom with their camera running? And there are plenty of things a cop rolls up on that recording probably shouldn't be done. They need to be able to turn off the camera in such cases. There will be occasional cases where the situation changes. (They roll up on a naked rape victim, while investigating it turns out the rapist was hiding rather than gone.)
1: backbuffers are great. Current body cams should have them. Older cameras before the bodycams era did.
2. Put the big fucking babies in diapers for all I care. The camera stays on. And activate storage dump on many events: getting out the car, drawing the weapon, drawing the pepper spray, getting a call.
I don't think you understand my point #2. It's situations where others would object to the cameras that I'm talking about. And there's no practical storage dump like you envision--that takes too much bandwidth.