ADDED: There is a degree of I know cheating when I see it. But, absent specific instructions to the contrary, spell checkers and grammar checkers/recommenders seem to fall pretty clearly into the assistive tools category whereas GPT-3 does not.
Legalities do matter here. The definition is “taking someone else’s work.” AI (currently) has no rights and no ownership. It is an “it” and not a “someone.” You are using a tool, an advanced tool that should be dealt with on a case by case basis. IOW, “don’t bring a graphing calculator.”
But it appears we are on the same page. If it’s directly against the rules, that’s different (like bringing an advanced calculator to an elementary math exam). But it def isn’t plagiarism.
GPT-3 is constructing entire narratives from prompts. It operates at the level of multiple sentences and paragraphs, to the scope of an entire essay itself. Based on a provided prompt, it generates an essay.
By your same logic, an instructor assigning a class essay would be considered the author of those essays as that instructor had generated the prompt.
Moreover, as a matter of pedagogy, where a course either assigns group work (such that multiple students are contributing to an essay), or requires drafts and reviews with an instructor, TA, or other students as part of the overall essay-writing assignment, those become acknowledge parts of the process. In the case of the draft / review / revise cycle, it would seem that that is one way of putting a check on AI-generated essays, at least for now, as though generators can follow prompts, they're rather more limited in taking criticism well, and adapting to it.
Hol up, didn't you just describe academic research?! Sounds like we're on the right path here.