Police often get calls about domestic disputes that require them to go inside houses. Would you want a video from inside your house to have to be made available to the public by law?
SWAT team about to enter a home or use a "no-knock" warrant? They aren't allowed to turn off the camera there. Same for the vast majority of the other police actions that happen in public.
Unfortunately, I'm sure the cops will find some loopholes to abuse this, too, and turn off the cameras when they aren't supposed to, especially if nobody ever punishes cops for such actions (which they don't), but I'd probably still take that over making all footage private by default. In many cases the cops doing bad things would still get caught because they forgot to turn off the camera or didn't think ahead to do it.
Either the cameras are on all the time, or you can forget about them.
And then back that up with career advancement metrics designed to motivate individual cops to ensure their camera gear is working and reliable. Missing 2% of your bodycam footage a week without having made a documented effort to get it repaired/replaced is a warnable offence. Missing more than 5% you get your pay docked. Miss 5% more than twice in a calendar year is a sackable offence. Any claims made in court about events that happen while your bodycam footage is unavailable remove your usual police department protection, and expose you to civil suits that you're normally shielded from. If a civil suit rules that "in the balance of probabilities" you were wrong, you become personally exposed to damages.
As the law & order types love to say "If you're not doing anything wrong, you've got nothing to fear." It cuts both ways.
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.
Have you given any thought to the implications of wearing a camera for 8-12 hours a day? Cops need to use the bathroom too... Body cameras will always need to have the ability to be turned off.
The rest of the time it should be kept and unwatched. Ideally, I also think it would be best if everyone had to give sworn statements before they knew what was or wasn't on video (so nobody gets to change stories to match...) and that the video was stored, archived, etc. by an outside agency that doesn't answer to either party. But I wonder how much of that they'd be able to accomplish.