The "Why" has to do with increasing market size for Amazon. If they can deliver small items to you in 30 minutes with no delivery fee, all sorts of things you

The "Why" has to do with increasing market size for Amazon. If they can deliver small items to you in 30 minutes with no delivery fee, all sorts of things you need in the very short term stop being a trip to the store and become a drone shipment from Amazon. This opens up HUGE markets for them. I also think people are thinking of the distribution network a bit wrong. They can keep a large number of items in a small space, particularly if the warehouse becomes sort of like those old magnetic tape storage robots you used to see. You could condense an entire Wallgreen's (almost every item of which is drone-able) down to a large room. So the practicality of having a number of them in a city becomes more and more viable. Of course you'd have software optimizing for the most common items on a per-distribution center basis (I bet Amazon already does this on a macro scale). Add in long-haul robotic trucks feeding the distribution centers and you can see where this is going.