What is the basis for an unfair labor practice claim brought by an employer against a retail store?

The bedrock on which our decision in this case rests is Hudgens v. NLRB, 424 U.S. 507, 96 S.Ct. 1029, 47 L.Ed.2d 196 (1976). There striking warehouse employees of a shoe company picketed in front of one of the employer's retail stores. The store was located in a shopping center mall that housed sixty retail stores and was surrounded by a large parking lot. Four picketers carrying placards patrolled the area in the interior of the shopping center mall adjacent to the entrance to the shoe store. The owner of the shopping center threatened to have the picketers arrested for trespassing if they did not leave, so the picketers left and subsequently filed unfair labor practice charges.