They get a percentage of every transaction (small, but it is about the aggregate for them). Mostly for Apple this is about pushing people who are using ApplePay to back it with a credit card that Apple gets a piece of, rather than the one you already have a piece of. So they get the small chunk from being in the loop with ApplePay, and now a small chunk from being the sponsor of the card.

Apple has been very focused on not being a big-data company. Any sort of back-end shenanigans would be so damaging to the brand of information safety that they have been building as to be unworth any potential profit.