Except that businesses are generally considered "public accomodations" & so the owner's rights to regulate behavior are not absolute. This may not prevent them from telling an employee how they must behave, but what if a customer parked on the lot in a truck flying the Stars & Bars? Bet that BJ's wouldn't get too far if the customer didn't voluntarily leave.
Not so. Public accommodations are considered public accommodations -- that includes hotels, restaurants, retail stores, and other businesses that extend an open invitation to the public. Even those businesses can refuse service to a customer who they believe is acting inappropriately or regulate an employee's conduct on the job.
Most business premises are not public accommodations. The public may not enter many office buildings, or park in their parking lots, without permission.