Festival’s move to ‘cash-less’ a sign of things to come?

Chattanooga’s Riverbend Festival has announced it will not take paper/coin currency for purchases. I just examined a dollar bill which reads: “This note is legal tender for all debts, public and private.”

Questions: a. If a vendor hands a customer a drink, hot dog, etc., before the customer has paid, and the customer takes a bite and then offers cash, which is refused, is the deal done?

If knowing this policy, a person doesn’t like such rules and decides to bring his own refreshments, will they be forbidden from entering? Can they be refused if they have already obtained a ticket? b. If said person is cagey enough to bring in concealed goodies, can they be legally searched?

China already is moving to become a cashless society (payments completed by phone). Where are we headed?