A lot of airlines charge a fee when using a credit card. For instance, Brussels Airlines :
Transaction fee
A transaction fee of 5€ will be charged when you pay your booking departing Brussels with your credit card. Debit methods of payment and Cash-Ticket are available and are free of charge.
I don't see why Ryanair is always pointed out as the bad guy. Moreover, Brussels Airlines and quite a few carriers still levy a prehistoric fuel surcharge, whilst there is none at Ryanair.