Google is just continuing their moat/scorched earth strategy. Google's bread and butter is advertising, everything else is just a moat to protect that. Search, Chrome, Android - they all help Google's advertising bottom line. If Amazon built an Android-based Kindle, Google would be tickled pink.
Motorola Mobility was a strategic buy. Predominantly, patent protection against Apple and Microsoft. But secondarily it helps them secure a permanent Android vendor. Buying Amazon doesn't really further their goals or business model. Amazon is a distribution juggernaut that is orthogonal to Google in just about every way (except AWS)
Seems there's been quite a few Android devices that have shipped with much of the Google services stripped out, and replaced with Bing/other vendor apps. This was going on at Verizon Wireless, at least for a while, and is again occurring with a recently announced Android tablet.
I think an acquisition of AWS by itself would be a strategic move for Google. I could see some interesting mashups of Google products and AWS. One that comes to mind is an S3-backed version of Google Docs, or using EC2 to do advanced analysis on data in Google Spreadsheets.
Fun fact: The Barnes and Noble Nook is Android-based. So Amazon would almost be behind the curve if they went for an Android-powered eReader.