When is a banner a picket?

The effectiveness of union tactics depends on many factors. Take for example the bannering and leafletting of businesses to protest some union objective. Thats what the Carpenter's union is doing in this California town. The National Labor Relations Act, since Taft-Hartley, prohibits secondary picketing. Thats where a union targets a customer of the employer with whom the union has a labor dispute. Its illegal to picket a "secondary," and the NLRB can enjoin the activity and a union can be sued for damages.
But hand billing, as compared to picketing, is lawful if it is peaceful and conducted on public property. As the linked article notes, the Carpenters are also using a banner, and the NLRB is placing those charges filed against bannering in abeyance until then Board makes a decision. The issue? Whether bannering is akin to picketing and subject to NLRB regulation, or whether the First Amendment protects the activity. That pesky 2 member Board situation just keeps getting in the way.