Use of the images can be abusive even if the creation of them wasn't. Revenge porn is an obvious example. Even legally and consensually created images can be used to abuse someone.
I suspect there's a transformation through intent.
This does lead to poor policing (like the famous Google banning man for taking photo of his child's genitals to send to doctor story).
It appears that in present-day English people use the word 'abuse' to mean more than physical abuse. If you're a prescriptivist that may upset you, but isn't that the nature of being a prescriptivist? Does it perhaps beg the question "why be a prescriptivist?"? Aren't you, perhaps, literally standing against the world?
I want to reserve the phrase "child abuse" for real child abuse, so that it gets taken as seriously as possible and not diluted. I think that is a sufficiently good motivation to stand on this hill.
Also wouldn't that logic apply to pictures that were originally taken openly and innocently?