If Amazon is going to achieve 30-minute delivery for items of that ilk, it will need a lot of rather large warehouses in many, many places, just to keep stuff within 16 km of everyone.
You forgot to enable corporate speak on your babelfish. They've no intention of being to deliver to even the majority within 30 minutes. They said
Amazon adds that it hopes “one day … to deliver packages to customers around the world in 30 minutes or less.”
Which roughly translates to - We're going to have depots in various locations worldwide so we can claim global reach. But to actually benefit from our 30 minute delivery time, you'll have to be one of the relative few who live close to those locations. But look, drones, cool eh?
Drones may be relatively cheap, but running warehouses isn't and if they want to achieve 30 minutes delivery to the majority, not only will they need more warehouses, they'll need to hold more stock. All of which costs money, so they'll just tell everyone about their new delivery service and you'll find out you're not eligible when the postie arrives