Here's the thing. In iOS 1-3 (known then as "iPhone OS"), apps could only get pictures from the user through the standard camera UI/photo picker.
But developers were complaining that it was not flexible enough and that they couldn't customize the view to overlay on stuff on top of the camera view, like custom controls and/or stuff like labels for Augmented Reality, something that was already possible on Android.
So in iOS 4 (released in 2010), Apple added the ability for apps to overlay stuff on top of the camera view.
Ever since then, it has been possible for devs to completely hide the camera view by overlaying something over it.
I don't see any way to prevent that unless Apple removes this ability, which means no more third party camera apps with custom controls and no more AR apps...
And as I mentioned, this was already possible on Android before iOS 4. Why is this suddenly news and why these proofs of concepts only seem to target iOS?