... Some merchants such as SweetGreen, a salad chain, refuse to open their registers for cash, telling customers they can pay only with mobile payments or cards. With some newer vending machines, only a card or mobile wallet will get that cold Coca-Cola to roll down the chute. The stance may appear un-American -- after all, currency is considered legal tender for all debts or dues -- but the Treasury permits private businesses to set their own policies, which means going cashless is fine with Uncle Sam. "What we've seen is a push toward electric payments because of convenience, especially...