They already have taken out browsing capability for new Kindles over a 3G connection. They could save money by not having a 3G capability on new Kindles, even for purchasing books, but that would leave those without WiFi access out in the cold. Eliminating 3G for purchasing books might cost them a lot more than having 3G for purchasing books, because without 3G, there are a lot of people who just would buy less books.
Do other company's readers have 3G? If so, people might just switch readers if they dropped 3G entirely.