Because card paying members may choose to shop the store where they don't feel like they're losing.
Ie; gas station 1 has a 3c cash discount. Gas station 2 has no discount, but you don't feel like you're not getting the discount. Gas station 3 can even compete by having a 1c credit discount.
Ie; gas station 1 has a 3c cash discount. Gas station 2 has no discount, but you don't feel like you're not getting the discount. Gas station 3 can even compete by having a 1c credit discount.