Theory is that credit card's charge a fee that can be passed on to the customer. People paying cash should not be paying a hidden charges built into the price. So
cheaper for cash buyers. A retailer, including airlines, must advertise a price that can be paid without a surcharge.
https://www.accc.gov.au/business/pri...ent-surcharges
Some retailer will charge a higher % fee for Amex compared to Visa/Mastercard. That's when I use my Visa cc.
Major supermarkets, fuel retailers, Target & Macca's do not charge a surcharge. The more up market the retailer is the more likely they will charge the surcharge. Airlines do charge the surcharge.