Cameras can malfunction. Buttons do malfunction. Having unexpected missing footage will be perfectly normal if there is a button that is used for breaks etc, because the police will have a reasonable explanation for why the button press didn't register (like "I was having a donut break, I was stepping out of the car, it looked as if someone was pulling a gun, and at the same time the police radio reported of a warning of an armed suspect, I could not stop there to confirm that the camera went on, I just ducked behind the car.")
There's a wider culture issue with American police organisations, and my understanding is that the situation varies wildly across different states and cities. I don't think that adding complex camera usage rules will help a lot.
I'm not American, and in my country of 5.5 million people the police use the gun about 50 times a year; in this context, pointing gun at someone counts as "using" the gun and needs to be reported. In 2013 the police fired six bullets (training shots are obviously not counted, but warning shots are).
And that six bullets is not per one policeman, they are for the entire police force of 7800 police officers.
And yes, the general public here has almost as many firearms per capita as the Americans do. It's just a different culture. Not one without violence or homicide, but still quite different, from the police point of view.