I think there's a line somewhere between outright plagiarism and lets say, borrowing inspiration from something else. Now that's where I can at least see the anti-AI argument, "but it can't be inspired!" Sure, but... I really think it comes down to nuts and bolts, there's a point where it may be impossible to tell if something was AI generated or not, so using "Not human" as a basis for if it's plagiarism or not is foolish.
If I write a story about a real-life character getting sucked into a cartoon world... am I plagiarising "Who Framed Roger Rabbit?"... or "Cool World"? No, not necessarily. Even if I use alot of the same words, i'm still not necessarily plagiarizing. Directly copying the same intellectual property is, and that's not always incredibly easy to define... which is why such thing is generally handled in courts. I don't think there is a universal "gotcha" for it save for a literal copy of not just individual words, but the idea as well.
"Created by AI" used as a measure for declaring plagiarism is... or at least will soon be... a moot point because one would have to prove that such a work was created by AI, and that will become increasingly difficult to impossible in relatively short order.
