Plagiarism by default is unauthorised so I think the title should be "AI is just authorised plagiarism". It's authorised by the markets, the governments and the society at large.
What makes you say that? Which governments? What society?
The current US government is not representative for governments out there in the world, you know.
While there are no hard boundaries (and the attribution guardrails depend on the situation), people of course loosely--and even not so loosely--use information, ideas, and even expressions from others all the time and that's considered pretty normal. And, if you don't want that to happen, don't publish/disseminate something.
Of course, if you quote a paragraph in a book, you're generally expected to attribute it.