A professor at my school was known to sell his own books as mandatory material to pass his class, with exams aimed more at the content of the book than the actual content seen in class.
Two years ago, his last book released with a digital version only. He put it on a platform full of DRMs and is now renting it to students. Like, you pay it for one semester through an online shop, and then you lose access to it. He was apparently also checking during the exams that everybody had a legal version on their computers.
I honestly can’t understand why it’s legal and authorized.