The USF honor code, available at https://myusf.usfca.edu/academic-integrity/honor-code defines plagiarism as the following:
"Plagiarism is the act of presenting, as one's own, the ideas or writings of another; plagiarism, in any of its forms, violates academic integrity. While different academic disciplines have different norms of attribution, all strive to recognize and value individuals' contributions to the larger body of knowledge. It is the responsibility of students to consult with their professors in order to understand the norms of attribution in each discipline and area of study."
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 USF honor code, 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!