i think they don't accept them yet because it is against their TOS
you might be right, also you need to provide your billing address and your credit card information so it would be really hard to pay with bitcoins
With the credit cards, they can verify your address easily. But with the bitcoin, there is no address information.
What about the destination of the items as a source of tracking? Or maybe Amazon could do ID/selfie verification if buyer wants to pay with Bitcoins