Common law is created by judges. Police depts. are statutory and created by legislatures.
What is unconstitutional here is that the 2nd Amend. was again ignored for purposes of granting fictional priviledges to a particular labor group when addressing full faith and credit problems. If the law is to have any validity in protecting rights and insuring those rights are respected across state borders, then the law should have honored the 2nd Amend. w/o regard to occupation. Congress should have explicitly applied it to the States, w/o waiting for a court decision incorporating it by the 14th Amend.
I believe that Lysander Spooner had some ideas on that subject.