If you can give some vague prompts to the model to obtain something that is close enough to a significant chunk of the work that, had it been written by a human, was susceptible of being considered plagiarism... then I'd say the same laws protecting from plagiarism should operate there.

Perhaps, but that's not even remotely what's being accused in this case. They're asking ChatGPT for a summary of the book and it's generating a summary a couple of pages long. Nothing is even close to verbatim, and I don't know enough about any of the books to know if those summaries are even accurate. In my experience ChatGPT often ends up hallucinating a lot of details when asked stuff like this.