" No assault on bus captain "

I was on the bus the other day and It was a long journey so i decided to look at some of the posters that were put up and I came across a sign that says, " No assault on bus captain. " 

Seriously? No assault on bus captain? Why do they need to put that up? Isn't it common sense that it is wrong to assault/ attack someone?! -__- 
I'm sure many of you have also noticed that sign, and you might be as puzzled as I am. 

I really don't understand why we need a sign to remind us that it is wrong to assault someone. It's an act of violence so it is naturally wrong. 

I know that there had been cases in the past whereby people actually attacked the bus captain for one reason or another, but by putting that sign on our buses, what does that reflects on us? If foreigners or tourists were to see that, what impression will they have on us? Will they think that we are a bunch of barbarians who resolve problems with violence?  

I'm not hating on the advertisers who put that up, I am just puzzled and bewildered by a sign that tells us the obvious. But I guess if we really need that sign to remind us about the assaults on bus captains in the past and that it is wrong, then I have no comments about it ._.