Is it legislation or the contract with the credit card? My understanding is the contract to take ie VISA has terms that you cannot apply a discount for customers using other payment methods (ie cash or someone else's card). There are a few places that don't have those terms (mostly government where often a card does cost more to use).

What people forget about these fees is a credit card is cheaper to take for the merchant. The credit card is never counterfeit money. The clerk never takes money from the credit cards, nor does the manager counting it (I wasn't in retail long but I saw both). You never have a robber come in to take your credit card money. Even when all goes well, you don't pay the clerk and manager by the hour to count all the cash twice. You do have some risk of taking a stolen credit card, but overall it is cheaper for the merchant to take credit cards and that savings should be what pays for the card costs (I have no idea how to count the different costs to see if that is true)