Bitcoin is to volatile for Amazon. I think once more small business's start accepting Bitcoin then the "whales" of the business world will begin to move in. Just look at the crash earlier this year from $4000+ to $2500. Let's say that's roughtly 1/3 of it's value it lossed.
How would Amazon be able to explain to it's investor it's lost 1/3 of it's value overnight due to accepting Bitcoin? (I know it's an extreme example as most sales will be in fiat, but I would think the point is still valid)