It's not entirely new! In 1975 during labor negotiations the police detonated a bomb on the mayor's yard, partially damaging the front door, and left a note saying "Don't threaten us":

https://cdnc.ucr.edu/?a=d&d=DS19750820.2.20

Since then, the SFPF have always had a culture of being above the law. The monopoly on legal violence thing can be taken a bit too far.