There are many theories — for instance, this is a way to put pressure on the FAA (very plausible.) There are many legal issues around drones, and I think being pre-emptive isn’t such a bad strategy. Others might think that Amazon is trying to get a lot more holiday mindshare (not that it needs it) or just that it wants to divert attention from some possible future bad news?
I think all three are very real and valid reasons, especially considering the negative vibe you get when mentioning the word “drone” these days. The 60 Minutes interview is worth watching as well.
Update: Bill Gates isn’t very impressed.