I will answer the question you said I didn't answer--what would happen?--and then I'll come to your present question.
What would happen if nothing is done in terms of price regulation? The trend is very clear. From the empirical data, the percentage of transactions accepted by cash or cheque is going to continue to decline. Some day, in the not-to-distant future, I believe cheques and cash will disappear. That's what I believe, based on the empirical data. That's point one.
Number two, in terms of Visa and MasterCard, I'm assuming we will not regulate prices in that section of the economy. The phrase I use with my own students about wage and price controls is that you cannot regulate, put price controls on one sector, because all the sectors are independent, or, to put it more colloquially, you cannot be half pregnant. It's all or not all. You cannot regulate one sector of the economy, because they're buying inputs from other sectors of the economy. The credit card companies hire people. They buy technology inputs. They buy broadcast wires, Internet wires. So because they're interconnected with everybody else, if you regulate the prices in one sector, you have to regulate everybody.
In terms of the third point, the merchant does not have to accept credit cards or debit cards. If they feel so strongly that this is really unfair, they can demand that every customer pay cash. They could have said to me when I bought my dishwasher, “You have to pay cash.” And I could have said, “I'm not going to patronize your business.”