Cash discounting was blanket permitted in all 50 states under Durbin. [1] It also de facto invalidated any clause in a merchant agreement that precluded this practice.
Only places I've ever seen do this are gas stations. And I've calculated the discount, I've never seen it be more than the cashback I get on gas purchases from my card. Usually, it is way less. And that is without getting into losing the time saving/convenience of paying at the pump.