Under American labor law all strikes involve the legal use of private coercion, and many of them involve outright violence. A picket line has one purpose - to prevent, or to try to prevent, replacement workers, customers and suppliers from engaging in voluntary exchange with strike targets. It is a physical barrier to the property of the strike target, and anyone who crosses the line is in danger of physical harm and harassment. Last summer's Teamsters strike against UPS illustrates the point. In any other context a picket line would constitute trespass.
Many people wink at coercion and violence in strikes while they condemn both elsewhere.
I think that is because they mistakenly believe that strikes benefit workers. The late W. H. Hutt, a leading 20th century free-market labor economist, taught that any gains received by striking workers or union officials are ultimately borne by consumers and other workers, not by employers and the investors they represent. Strikes do not benefit workers in general. There have been many empirical studies that support his hypothesis. Now we have empirical evidence that indicates that even striking workers gain very little, if anything, from strikes.