For the most part, professors have to use books that the university bookstore can obtain. Since the publishers were always bumping up the edition, that meant using the new edition. When (and where) I was going to school, most professors would turn a blind eye to students using an old edition. Many would even go as far as supporting students with the old edition. A few would recommend entirely different books if they felt that they were better. I even had one professor who paid students for finding errors in a book that he wrote, even if he knew that the student was using a photocopy of his book.
Very few professors are opportunists and most would prefer an open culture. They are simply stuck with the rules of a system that preys upon students.