Hasn't Amazon implemented their own?
As far as I know they use GE as the issuing bank, they may have other deals in place.
As far as I know - but this may very well be wrong or at least out of date - amazon does not have a banking license and the network used is VISA.
This seems to confirm that:
I would guess Amazon have looked at replacing as much of the payment chain as they can and done the parts that are profitable to replace. But they have a long way to go before the enormous hassle of becoming a bank is going to be attractive and possible.