It is ethical for unions to verbally criticize nonunion workers who "cross the picket line" and perform the work otherwise assigned to the striking laborers. It is not ethical for unions to physically attack or even verbally threaten nonunion workers, or "scabs." Freedom for workers to organize is a fundamental American and international right, but that right does not allow organized labor to use violence in pursuit of its agenda. Striking workers establish picket lines for the purpose of publicizing their grievances against management while forcing production to halt. It is always the hope among such workers that prospective customers, vendors and others will honor their picket line and cease any interaction with management of the company in question. Fairly or not, however, those same picket lines function as potentially-threatening deterrents to customers and others who might otherwise be inclined to support the company's management in its negotiations with its workers. The mere sight of angry laborers is sufficient to deter many from crossing their line, which is why management employs security guards to escort "scabs" to the factory or service line, and which is why law enforcement agencies might be mobilized to similarly protect the rights of nonunion workers to enter the corporate compound.
While it is unethical for unions to physically attack "scabs," or to threaten them with violence, one can, conversely, question the ethics of those nonunion workers choosing to cross picket lines and take the jobs of striking workers. Again, workers have a right to strike (in most instances), and the appearance of nonunion workers undercuts the union's ability to negotiate revisions to existing agreements or to establish fundamental rights in the first place. Whether the union's grievances are legitimate or not is not for us to decide. Unions have their excesses, and have engaged in extremely unethical behavior over the past century, but they exist for legitimate reasons born of management abuses that forced workers to organize. What is not ethical from either direction, however, is the use of violence to gain advantage.