It's odd - I'm not sure if a phone would drive more amazon services. What's their motivation? They already see what I buy, when, and in many cases for whom, but they don't really seem to use that information very effectively.
I see some targeted stuff, but not as much as I would expect given my ridiculously long shopping history. A phone would allow them to know more, but they don't seem to be using the data the have...so the extra data would be pointless.
They're not like google, in that their mobile is not a stalking horse for targeted ads. They're not really like Apple, since they don't really make high-end hardware. Launching a cellphone because you're feeling left out isn't really a great business case. Amazon doesn't really need to control the experience, since you can buy anything on amazon on other platforms.
Maybe they got a good deal on bulk minutes and cheap hardware, and want to pass the savings onto the public?