Since the gigabit+ links they are paying for are symmetric, that is, they can move the same amount of data in both directions, I will make the assumption that A

Since the gigabit+ links they are paying for are symmetric, that is, they can move the same amount of data in both directions, I will make the assumption that Amazon's "outbound" traffic is less than their "inbound" and thus, an increase of inbound up to a certain point costs them nothing. Also, transfer-in means that in order to get any use out of it, you now have to transfer-out (another charge). Not being critical, just pointing out that this is a bit of a "give the razor for free, sell the razor blades" strategy.