It's not law (though there may be state/local laws also), it's the merchants' agreements with Visa/MC/Amex/etc. Cash discounts are allowed - you see them with g

It's not law (though there may be state/local laws also), it's the merchants' agreements with Visa/MC/Amex/etc. Cash discounts are allowed - you see them with gas stations quite frequently - but minimums and surcharges are disallowed and the merchant will almost certainly get into trouble with the credit issuers if it's reported. It's almost entirely small businesses that try to flout their agreements - you'll rarely see any major business try because of the consequences.