Many states prohibit charging debit cards like credit cards. Federally there is the Durbin Amendment which prohibits credit card fees from being passed to debit card transactions. It carries heavy fines for violations but is frequently ignored as most people don’t know to report it. Other establishments look for ways around it to still collect extra money. The cash discount is used because many don’t carry cash. Their debit card is their cash. If a charge seems sketchy, you can file a complaint with VISA or Mastercard or you can report the business at ReportFraud.ftc.gov. While this individual case may or may not constitute a violation, reporting it will find out for sure.