This is just my .02 but the big question is what kind of company is amazon currently and what kind of company would they like to be? Getting into the transportation and logistics business because it is a cost center for your business may make sense, but is that really the business you want to be in? Do your core competencies translate well to that business?
I'm not quite sure I believe the justifications put out so far by Amazon, but perhaps they are playing a longer game (They want to be a logistics company???!!)