Gas (Paying Cash)

Gas stations are giving discounts to customer's that pay with cash. Gas station owners aren't making profits from gasoline sales since they have to pay for credit card transaction fees. I don't understand this. I have my money direct deposited & use my bank card (as credit) all the time. It's very rare that I have cash on me. Also, don't you have to pay first when using cash? That won't work for me. Most people don't know, but when filling up their vehicles, when the gas pump automatically stops... you shouldn't put more in to make it an even dollar amount. When it stops, your gas tank is full & it's better for your car.