The credit card processing chain is quite complicated, with banks, card companies, ISPSs, processors and merchants all having fairly clearly delineated roles. A merchant - no matter how big - implementing the whole chain is unheard of.
Visa, MC and other cc companies care a lot more about their reputation than they'll even care about a company like this.
Much larger fish have been put on the bbq over less substantiated claims of fraud.
I've got an Amazon credit card (in the UK) and it's a Mastercard run by MBNA.
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.
It's not nearly enough. Some major conglomerates (If I recall correctly then GE, GM, Samsung and others) do stuff like that in-house by creating bank-subsidiaries in their largest markets, but I'd guess that it would be reasonable at 78 billion revenue, not 78 million.
That's just to become an acquiring bank, to save on exchange fees.
Ultimately VISA determines who gets to participate in the VISA credit exchange network and will fine acquiring banks who participate in things like this. No one can force their way in and avoid responsibility -- period.