I would think that Mr. Crowley is hoping that, eventually, other credit card companies will want to do the same thing; allowing them to generate income. American Express is in need of an advantage over Visa and other major credit card providers, while Foursquare needs someone with the capitol and resources to begin a new venture.
Good Idea Foursquare! Would it be smart for American Express to build an exclusivity clause with Foursquare for this service?