I saw a few of these "vandalized" billboards until I looked it up and realized it's just an advertising scheme. It's so fucking stupid because I thought these fuckers got what they deserved until I realize they themselves did it just to get people to look at their ads more and create discussion.
Which is an interesting tactic because like you, I thought the law firm was scam and people hated it enough to risk jail to deface. So now I know of them but think badly of them due to association. Great job, PR team!