I don't disagree (in fact, I think it probably is the case that they get a lower rate for being exclusive), but just to add: Costco also has a partnership with

I don't disagree (in fact, I think it probably is the case that they get a lower rate for being exclusive), but just to add: Costco also has a partnership with amex issuing their Costco branded credit cards (which isn't a store only card, it works out in the wild), and Costco gets some portion of the revenue whenever that card is used. Restricting you to amex also has a benefit of increasing the likely-hood that consumers will sign up for the Costco cc.