The problem is that "derivative work" is NOT a legally clear term.
So this "clarification" may not stand up in a court of law, but it places distributors on clear notice as to the copyright holder's understanding of the law.
If a term is legally ambiguous, but the defendant knew up-front the interpretation the plaintiff placed on it, then the defendant cannot argue "innocent mistake". They *have* to argue "plaintiff is wrong", which is a lot harder. The "as I understand the law" defence is a lot harder if the plaintiff says "but I told you that's not the way I understand it".