I am not familiar with every country's rules, but I do wonder if this is a case of 'without one's consent' or perhaps a retailer's preference to charge in the currency their payment processing system deems best? Additionally, are we certain this is imposed by the airline? Or is the bank that issued the card forcing the transaction into the local currency at their preferred rate? This practice sounds to me like something a bank would do.

Is there some sort of rule that requires a retailer to offer payment in a specific currency? Yes, some retailers offer at the time of payment what currency, but is this an obligation?