Why? When you purchase a book at a store and return it, do you not get your full money back? How is buying a virtual product any different?

I can go to a bookstore, buy a book, read 10% of it in my car, go back inside and return it for 100% of my money back, no questions asked. A system that punishes consumers for returning items would die out fast, especially since there has to be a legitimate reason for returning now. Neither the reader nor the author should have to pay Amazon when a book gets returned.