Why would a gas station offer discount for cash for any reason other than to avoid it going through the books and hence avoid obligations like paying tax or their staff a proper wage?
Card payment processors charge the merchant some premium. Usually their contract with the merchant prevents passing the fee along to the customer. A cash discount could be a way to get more customers to pay in cash, which avoids the credit card company's cut. This is probably violating the terms of the merchant/card company contract, but I dunno, hard to resent the merchant here given the power imbalance between them and the card processors.
How would using cash would be an effective way of denying the gas station's staff a proper wage? I suspect most gas station staff is hourly, rather than... I dunno, tipped or commission.
Cash could certainly be a way of avoiding some taxes.
A lot of places do this, whether it is "allowed" or not. But my understanding is that after much push and pull between merchants and processors, most processors do allow it.
The gas stations would have never had any credit card uptake without it and people have to buy gas and are extremely price sensitive so would have continued shopping at cash only gas stations.
Unlike say a clothing store where the convenience of credit overcomes the price sensitivity, you can defer the purchase and the merchant has higher margins.
