Amex is one the most expensive, if not THE most expensive credit card out there, I carry a Platinum it's pretty nice, there're lots of privileges, such as VIP rooms at airports, special menus in some restaurants, stuff like that. ALSO, stuff that you can easily live without, so nice to have it, but whatever if you don't lol.
I'm pretty fond of that kind of promotion however, it advertises the product (both game and credit card in this case), and it's not "damaging" the player like, for instance, getting an exclusive ultra-powerful in game weapon (in multiplayer that would be unfair, to say the least). Other credit cards like Visa and Master Card have similar behavior after all, here in Brazil, depending on the cinema company, you might get to pay only half a ticket, etc.