Should have asked for the Store Manager. He would have either reversed the clerk or confirmed the policy of that store.
If reversed, you'd have been good to go. If not, get mgr's name and leave - call corporate the next day.
You were breaking no law, so there is no fine. Store policy is not a statute nor ordinance - the most they can do is ask you to leave. If you do not comply, that is entirely a different matter.
Clerks with faulty knowledge are a PITA.