Perhaps surprisingly, whether picket lines are lawful is often a matter of legal contention when there's a strike. Company owners argue that they are obstructive (which is at least partially the point), and courts often grant injunctions or restraining orders placing limits on the size, location, or even existence of picket lines.

A coal company in Alabama recently won a restraining order that fully prohibited a picket line at their mine: https://www.msn.com/en-us/news/us/judge-issues-restraining-o...