There have been disputes between AMEX and quite a few retailers (not just airlines) regarding the higher commission rate charged to retailers by AMEX compared to Visa and Mastercard in Europe. While this is typical Ryanair (AMEX presumably refused to lower their rates to those of Visa and Mastercard) it is slightly strange for hem to complain when most low cost carriers in Europe already charge EUR3-5 extra for using a credit rather than a debit card.