I've often wondered about this too. They've got to know the cameras are there. But in dangerous encounters when the adrenaline is surging, do they forget that because they're in the moment? I once heard a psychologist testify that when emotions are high, we do not "read" situations correctly (that's why different people can view the same accident, and all honestly have different recollections of what they "saw"). We aren't just dispassionate pieces of equipment. Our emotions affect our actions and our recollections.