De-escalation is part of why I'd want that kind of camera system - the signs would serve a dual purpose, A) notifying and hopefully at least heavily clouding charges of "he had no legal right to record, we weren't in a public place, charge him with wiretapping" (I'm in Illinois which is an all-party-consent state) and B) putting police on notice that even if they aren't using body cameras (or they're prone to "malfunction") there will still be recordings.

Given the apparent change in officer behavior that accompanies body camera use, perhaps having explicitly noted video and audio surveillance disclosed in advance will serve a similar purpose.