Plagiarism is to misrepresent something as authored by yourself. Whether you paid someone else for writing it or copied it without permission does not matter for the plagiarism charge - the wrongdoing is the misrepresentation.
It is not the same as copyright infringement - although it might be both at the same time if you copy something without permission and publish it under your own name.
Plagiarism is not always illegal. Publishing a book which is ghostwritten by someone else is not illegal AFAIK (as long as they get paid). But in education and research it is illegal - and they don't care if you bought the material fairly, the point is that decisions (grades, hiring, funding etc.) are based on the assumption that the material reflects your abilities.
So it is not really "theft of ideas", it is "faking credentials".
One of the issues that makes this murkier is that it is often only social norms and expectations that determine whether someone is misrepresenting themselves. In the meme world, the whole purpose is to share and modify slightly. There isn't an expectation that your post is original. In the academic world, every single word is assumed to be original unless cited otherwise. There are areas in between these cases, and often what someone calls a violation is merely the case of someone misreading the social norms.
I agree fully. I left academia because of exactly this reason.
While academics have high morale on not stealing more than four words in the same sequence, they don't have any issue putting their names first on some engineers idea.
It is not the same as copyright infringement - although it might be both at the same time if you copy something without permission and publish it under your own name.
Plagiarism is not always illegal. Publishing a book which is ghostwritten by someone else is not illegal AFAIK (as long as they get paid). But in education and research it is illegal - and they don't care if you bought the material fairly, the point is that decisions (grades, hiring, funding etc.) are based on the assumption that the material reflects your abilities.
So it is not really "theft of ideas", it is "faking credentials".