Many establishments reserve the right to refuse service, but that call should come from management not a cashier.
Among a staff of thousands, you’re going to have a bad egg. Wendy’s did the right thing. If I were in Wendy’s management I would have attempted to call that officer and offer a personal apology. That may have been done.
Reserving the right to refuse service technically became illegal with the passage of the Civil Rights Act of 1964.
Irony: The Mark of Quality Literature.