Amazon was targeting book readers who may not have been computer users, so they wouldn't necessarily have Wifi (or any Internet) at home. The cellular connection (which was free and not dependent on any data plan of the user) allowed the user to purchase and download books, and so paid Amazon for the very small amount of data used given that a typical book is only a couple of megabytes.
Home broadband connectivity was only about 2/3 of where it was today. The iPhone was just rolling out.
As much to the point, Amazon was also very much aiming for the use case of the business traveler who wanted to buy a book before hopping on the plane in an airport that probably didn't have WiFi and almost certainly didn't have free WiFi.
I won't say 2007 was the stone ages. But a lot of things we mostly just take for granted today didn't exist or were relatively nascent.