Alice and Bob were doing some shopping at Costco in the United States. Bob is Costco member, and Alice isn't not. The cashier, citing Costco's policy, refused to accept Alice's credit card: Alice was told that non-Costco members can only use cash and debit card. Only Costco members may use a credit card a means of payment.
Why would Costco refuse credit card payments from non-Costco members? I'm trying to understand if this is related to fraud protection, credit card fee reduction, or something else.