Saybrook University's Policy on Academic Honesty (Plagiarism) and Authorship, available at https://catalog.saybrook.edu/content.php?catoid=116&navoid=10878&hl=%22plagiarism#student-services-personnel (scroll down to subsection on "Academic Honesty (Plagiarism) and Authorship") defines plagiarism as the following:
The difference is cultural ethical practice versus law. In U.S. copyright law, it does not matter if you cite the original author/creator. Plagiarism involves, not just copying other people's work, but not giving them appropriate credit.
In short, it is not Western academic practice to copy other people's work without giving them credit. You can use other people's work, but you should quote and cite appropriately in order to be behaving, not just within accordance with the Saybrook Code of Conduct regarding Academic Integrity, but the ethical practices of the Western world.
On the flip side, if someone, for example, prints 30 copies of the current bestseller and sell them for $1 each because they're a huge fan of the author?? Well, that's not plagiarism, because the author is clear. That's copyright violation!