Probably easier to keep your trucks graffiti free if the owner is mobbed up. Nobody wants a casual visit from the pinkie rings.
CC
I gather the word got out that if you were caught spraying a union truck the justice was instant and painful. The situation was that it is easier to practice your art* on something else.
* Art. Yep, they think they’re artists. I guess if you can’t sell your own art, you can at least make someone else’s stuff look ugly.