So Cryanair would have to add £4 to all of their displayed airfares, but you could get it for less if you had the right card.
But what would be easiest is to change the law such that you may only pass on transaction fees and taxes; not make a profit on them. If they want to take a loss by having it at nil for one type of card then fine; but they can't charge you $10 when it only cost them $1.