I'd guess that Amazon gets very very low fee rates for payments. Amazon's not going to start competing with Visa, and it seems unlikely they'd be a bank, so I doubt there's a huge amount of profit to be made in taking over their own payments processing.
Why wouldn't they compete with everyone that is a material cost to them? 10 basis points on their revenues is already approaching $100M/year. My guess is they pay quite a bit more than 10 basis points too (spent a while looking for reliable source to corroborate this but couldn't find one).
AMZN seems to want to do everything they can to expand and lower their costs; I don't know why credit card fees would be excluded from this.
They could develop a completely novel solution over time. My guess is they will become much more serious about payments over time because of articles like this[1] and this[2].
If they did this, I would want them to branch out Amazon Payments to be a PayPal competitor. I would switch in a heartbeat. PayPal has incredibly high fees and unfortunately, is a necessary evil for me. I'm uncomfortable leaving any amount of money sitting with them.
