i think it would be more likely down to how much revenue they earn from the card, perhaps higher revenue they are willing to bear higher risk.
NAB is the issuer of the credit, i don't think they can pass that risk on to Coles or any other white label client. I think a business would want to outsource something like a credit card so you can outsource the risk Suncorp is a good example as they have a credit licence but choose to outsource credit cards to NAB.
Could be wrong, i work for a business that outsources financial services and one of the biggest draws for us is that we take on the risk.