It's vital to protect our "the advertised price is the price you pay" rules. Before these fee rules we had e.g. Ryanair advertising prices that were only available when paying with one rare credit card, and a substantial surcharge on all the cards people actually used.

I believe merchants can still offer a cash discount, which seems like a better way of handling it: cards shouldn't be subsidised, but advertised prices should be the prices when paying the way most people actually pay.