What rationale is there to have a law specifically about this? It seems to me like there's three contracts involved: The agreement between merchant/CC, consum

What rationale is there to have a law specifically about this? It seems to me like there's three contracts involved: The agreement between merchant/CC, consumer/CC, and consumer/merchant. Since these are all private parties, why does the government even care what contractual arrangements they make among themselves regarding how the form of payment affects how much needs to be paid?