Under American labor law all strikes involve the legal use of private coercion, and many 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 target's property, and anyone who crosses the line is in danger of harassment and physical harm. 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. Strikes do not benefit workers in general. Many empirical studies support Hutt's hypothesis. Now we have empirical evidence that indicates that even striking workers gain very little, if anything, from strikes. The big winners are union officials and the institutions they rule.