They're the same thing and used interchangeably. The sexual abuse is from the sharing lascivious pictures of someone taken without their consent, with the additional context that children are not understood to be capable of consent for sharing lascivious pictures. It is not physical abuse.
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.
What would you call whatever that japanese underage comic thing is? It features no persons and no one was abused yet it still illegal. I don't object to the term as applied, but I don't particularly feel great about the word 'abuse' being used for cartoon characters since it degrades the experience of human survivors.
Not in the U.S., federally at least, where the justice department's guidance specifies that CP is media which "appear to depict an identifiable, actual minor."
Lawyers and courts say child pornography because laws are not restricted to sexual abuse material. Drawn child pornography is illegal in many places for example.
It's the only term I'd ever seen for it, in any context, until the last couple years. And I'm not a 4channer. Pretty sure it's just another euphemism-treadmill thing. (not that I mind in this case, I think the new term's fine, and certainly not worth fighting over)
What's "the industry"? The relatively new industry in moderating Internet communication? I've certainly only seen the term "CSAM" coming from cops and prosecutors very recently, and they're rarely shy about using their jargon in public communication.
[EDIT] I believe you that it's the term in vogue now, to be clear, but I'm skeptical it was as dominant, even if present, until relatively recently. If it was, then that entire world only recently started using it consistently in public communication, certainly. But, again, it's also fine, I don't mind the new term.