That and the tests. Many teachers like the pre canned tests these books come with (please finish problems 10-50 even by the next class). Now it is online run by the publisher. Cheating is now even more rampant than it was before the code thing with many sites having the answers.

I would wait 1-2 weeks after class started to buy books. Especially if the teacher decided you need 6 of them to finish the class. It was usually pretty clear which ones they were going to use by that point.