Well, your problem was that you focused on the wrong issue. The question of public accomodation is a complex one. You are probably wrong on that issue, but no one would know without a lawsuit. What they would know is that a store clerk has absolutely no authority to tell anyone anything. He cannot act on behalf of the entity that owns or operates the store and he has no common law right derived from his status as an employee other than that which is the customary duties of a store clerk.
Placemarker
He cannot act on behalf of the entity that owns or operates the store...
Of course they can. They do it all the time. Owners are not always present and employees on duty take the place of the opener interpreting policy, making exceptions, answering customer complaints, safely removing cash and checks, filling shifts, sending people home earl, and calling the police on shop lifters and other criminals.