What exactly would Amazon do with the Bitcoins? They can't pay their employees or maintain their servers with it. Amazon would be the only major store taking Bitcoins. They would become bloated and Bitcoin would be worthless. Rick is right, it is too big and too soon. Economies need to cycle money and Amazon wouldn't have anywhere to put the Bitcoins after they have been paid.
Amazon can provide a world of good to bitcoin, but chances are that right now, they won't do it since it is too risky for them to do so.
Yes, they already have Amazon coin, but why not take advantage of the proven protocol of bitcoin, and the widespread holdings of / interest in the currency?