I've always suspected that much of Amazon's fulfillment process was a loss leader. It's hard to remember, but before Amazon if you ordered online you had to wait at least a week, and sometimes two or three or more for an item to arrive. "Six to eight weeks for delivery" was a popular cliché for a reason.

Because of Amazon, people these days expect to have stuff in a day or two or three.

Now that Amazon's killed all of its competition, perhaps it no longer needs to take that loss and can start moving to models that make more financial sense.