Why is it even possible to turn off or delete footage from these cameras?

I mean, police have a lot of privileged access to various locations. If they think that there's an emergency, they can enter your house, say. I think that I'd want to have the technical ability to have footage of that deleted.

EDIT: Also, while I can understand not wanting deletion to be available to an individual officer if the camera is acting as a check on them, I don't think that the main reason that police carry cameras is as a check on police. I think that it's partly because people make false claims about police officers (e.g. "you planted those drugs on me!") and if there's video footage, that can disprove that. I think that part of it is gathering evidence. And I think that part of it is as a check on procedure, to figure out how things go wrong.