As someone who has done a little bit of iOS development for fun, I wouldn't be surprised if this is an actual bug. There used to be 10 different ways to do the same thing inside iOS, and neither one was "correct."

Being a photo app, they probably figured out that having instant access to Camera is their top priority, and because you are allowed to access camera in the background and keep it on, there was no harm in doing so.

Of course one could also go deep and assume that Facebook is running all video through an ML that picks out faces, location, items, and whatever else they can grab around your surrounding, and then covert that into ad-intent, but however sexy that sounds from technical point, they are after all a public company and need to play by the rules. Unlikely to take on such a massive intrusion of your privacy, knowing that eventually it would get discovered.