Textbook Monopoly

A recent patent to halt students from sharing books, reinforces the monopoly that textbook publishers have on their intellectual property.  Wikipedia has all of the arguments for and against copyrights here.

The patent requires an access code for all digitized content.  So once I purchase a textbook online, as required by the class, only I can use the book.  This prevents sharing and the resale of used books.  I can understand that an author would want to sell as many copies as possible.  I can understand that a publisher will maximize profits where marginal revenue equals marginal cost.  I don't understand why students would not want to keep their textbooks after the class is finished.  The end user, the students, must not derive enough utility from the book to merit keeping it so why protect it?