Do you understand, then, the frustration of a merchant?
For example, suppose you've got the four-party credit card model, and let's say there's $2 off a $100 purchase, just for argument's sake, and the merchant signs an agreement with the payment processor. Suppose the amount goes up. You're a smaller merchant, and the amount goes up. You're not paying $2, you're paying $2.20, so you say to the payment processor, “Why is this going up?” The payment processor says, “Well, it's actually still 50¢ out of every $100 purchase; it's actually the interchange that has gone up.” Then the merchant says, “Well, I'll talk to the bank, because the interchange goes to the bank”, and then the bank says to the merchant, “Well, no, we don't set the interchange fee; the credit card companies set the interchange fee.”
Do you see how a merchant looking at the system, which Mr. McKay said was Byzantine, will find it incredibly frustrating?