- you cannot transfer AmznCoin to another person (they can only be used to buy stuff from the Amazon Android store)
- they have a fixed exchange rate where $1 = 100 AmznCoins
So, this is nothing more than a pre-payment method and a way of batching costly credit card transactions (along with a promotional giveaway). It's interesting to compare this approach with Apple iTunes. Apple considered the possibility of introducing their own "virtual currency" but they decided against it for usability reasons...mainly that people would have to always keep in mind an exchange rate. Apple went with the much simpler model that their "virtual currency" is pegged 1:1 to the USD and is simply called dollars. With this approach people don't have to think about exchange rates.
To solve the batching problem, they delay billing and batch multiple transactions together when they can, and they market and sell iTunes gift cards everywhere. It's an effective model and much more user friendly than forcing everyone to buy $10 or $20 worth of credit up front. They probably do pay a high fee on some individual transactions, but I'm sure they make up for it with the tons of $50 or $100 gift cards that they sell.
Yes, they're not a currency, they're credits, or a digital gift card.