Yes, I know what pointing accomplishes. But it's also very effective to communicate without marking up my screen in greasy finger prints. Everyone — everyone — who thinks that they can point at something on a screen effectively without touching it, is just fooling themselves. You will touch my screen if you point at it, and I'll know you touched it, because your fingerprint will be there, painted in disgusting finger grease distorting my beautiful pixels. Maybe not every time, but most of the time.

There is a place for prized asshole behavior. Frankly, touching someone's screen when it clearly bothers them is pretty damn rude, imo. If the only way to get them to stop is to make them uncomfortable doing it in your presence, that seems fair to me.

I'd react roughly the same way if someone rubbed their genitals on my screen while I was using it. It's that offensive to me. (Actually, I wouldn't mind the genitals quite as much. They're probably cleaner.)