I think that a lot of comments here are missing a crucial point: in the next few years unmanned vehicles are going to play a major role in most of the transportation landscape, whereas this is for people or objects.

Amazon here is trying to play the same role that Google has been playing for a long time with their self driving cars. They are setting a standard and pushing for regulations and laws.

It is not that difficult to think that the future of unmanned vehicles won't be as naive as people may think. There will probably be a combination of different sets of vehicles that would go around for large distances and/or sizes, and different vehicles covering smaller distances and/or sizes. They can be used in series to optimize the traveling salesman problem in terms of what's powering these vehicles.

It's not that different from thinking of an ant-like distribution system.

And not that different from the Google approach of crawling the web, then crawling the actual world with google street view cars, eventually moving into self driving cars. Except you'll also get lots of metadata with it, that can be used and sold.

Amazon is now part of the game, moving the attention to drones and self flying objects.