I think the reason he is getting a punishment in the first place is the traffic violation. Yelling "pigs" was just a cherry on top that helped when it came to coming up with his "unusual punishment" for this scenario.

Even though the act of shouting it in itself wasn't illegal, it was still not nice and disrespectful given the circumstances, hence why it was incorporated into the punishment. I am struggling to think of a nice "unusual punishment" in this traffic violation scenario that wouldn't incorporate the yelling part, but I guess I am not as creative as this judge.