There's probably also a bit of "Apple and Google have solar and fuel cell covered, where can we make an impact?" involved. Not a bad thing. We need to be investing and experimenting with lots of different alternative energy strategies.
I would like to see someone do a large scale implementation of marine current power (underwater turbines spun by currents like the Gulf Stream).
Solar is more suited for 24 hour power, which Amazon needs. It also costs less than solar, but needs maintenance, but if you're vertical integrated like Amazon then having your own low cost maintenance team is probably not an issue.