I'm sure Amazon would be happy to sponsor a little league team. Sponsoring a city in the same way is to then be responsible for people who don't even work for you. It's much better for Amazon to stake its flag in a city that is self reliant, but also fertile for new blood.

Microsoft and Boeing anchor Seattle next to Amazon, that makes Seattle a very stable and fertile place. If Amazon moved to, say, St. Louis? What else is there to balance it out? Budweiser?