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PCMerchants that want to accept credit/debit card payments have to sign an agreement that says (among many other things) they will comply with the rules established by card associations. So, the merchant would be punished, because the merchant is the one violating the card associations' rules.
It isn't like the MasterCard police show up if you accept credit cards and then try to collect more from customers seeking to pay with credit cards. But getting on the bad boy list (which is actually hard to do) makes it very difficult to obtain access to the credit card payment networks in the future.