From an economic perspective, there are two costs here. One is the author's cost, and the other is consumer's cost. The author's cost has been covered.
The consumer's cost is a transaction cost. It consists of the additional time, effort, and bother of getting a book from SW rather than from Amazon wireless. Different consumers will assign different values. However, if an author wants to get a book into the hands of a specific consumer, it can effect the probability that the consumer will actually download the book.
It's a mistake to project our own values onto others. For example, it's a mistake for me to think everyone else would rather pay money to get a wireless download than to go through SW.