"violating a California Vehicle Code section prohibiting the use of horns except "when reasonably necessary to insure safe operation'' of vehicles or as part of a theft- prevention system."
Using your automotive horn to express support for protesters isn't what it was designed for.
Everyone seems to have over looked the fact it was shortly before midnight. I would bet that there are noise ordinances also. If this is anywhere near a residential area and they were blowing thier horns shortly before midnight near my home. A ticket would be the least of their problems.