They are providing the service of making your money available for payment without you having to think about the logistics of moving the money (plus credit facilities, chargebacks, fraud protection (If I have a wallet full of money and someone spends it I'm down that cash. If I have a wallet full of cards and someone spends with them, I'm covered.)). It only seems fair that they get paid for their work.

Whether the amount they charge is worth the services they provide is another question entirely.