This is like renting a house and the owner saying "we installed a camera in your house, but we'll only look at the video, if someone screams 'help'"
Then someone says "what if they change it so it sends the video if someone says 'do you want some pot?'"
And your argument would be "wouldn't you just move if they did that?" - this argument is pointless, because it's not the "help" part that bothersome, it's the always recording camera in a black box, for which you never know what's recording and what's it sending to apple. Let's say hypothetically they do this, and modify the database to trigger on the "tank man" photo, and give that photo such weight, that it triggers manual review by the CCP observer (remember, apple data for chinese users is hosted in china) - you'll never know the image got flagged, you'll never even know that it was sent anywhere, but your social score will go down and you might "vanish" during the night. Imagine some new wikileaks happening... 15 journalists around the world get the data, NSA/CIA/FBI has hacked one of them and found some photos of USA doing something bad.... add those photos to the database, have apple scan all the phones, you find the other 14 journalists, and possible even the leaker who took the original photo. If this is expended to macbooks (which it probably will), even worse, because people keep more sensitive stuff on ther computers than their phones.
And all this for what? Child molesters and rapists (the worst of the pedos) create new photos and videos, that are in none of the databases, so you never catch them. Their direct sponsors (paying customers) get the photos first, and again, before they're in the database. So all you've caught is someone browsing 4chan.