You are clearly an American :) Also, Amazon would approximately double its profit if it were not forced to pay the 2% markup on credit card charges. And before you criticize coinbase for their 1% conversion fee circle is here with 0% :)
Amazon will accept your credit card payment and won't accept your Bitcoins, which is a pretty good indication of which payment method they feel is more hassle than it's worth.
Fortunately Overstock does accept Bitcoin already[0], so it's a pretty good indicator that there are other considerations too. Also, you can use Gyft[1] to spend bitcoin on Amazon and even get a discount - if there's enough business that another company can support itself on users you are not catering for, then there's definitely a business case...
