There is one legal problem with the AI-student pair: the student doesn't own the copyright on what was produced with the AI. Meaning, any work submitted by the student that was at least partially generated by an AI is legally not 100% produced by the student.
So the comparison with an open-book exam or using a calculator doesn't hold: if I search for information in a book, or use a calculator to compute a number, and make my own report in the end, I own the resulting product. I'm the sole author of that product. If I use ChatGPT, ChatGPT is the co-author of my work.
So, using ChatGPT is the equivalent of calling your dad during the exam and ask him to answer the questions for you. Is that really what we want to evaluate?