20 years ago, most of my courses after year two of university, the professors insisted that the newest edition of the book was required, as well as the three supplemental books that were necessary, even though they would never be directly referenced during the class.

I'm sure it was purely coincidental that they were the authors of the books. We were free to take up any complaints with the department head (who was the instructor), or the ethics committee (which included the instructor.)

While I'd hope that kind of shit has gotten better, I'd be shocked if it has.