It seems to me this is a potentially dangerous situation. If I am on the a par three green, why would I invite someone to hit a ball at me? Most folks can reach a par three green. How far off to the side of the green do I need to stand to avoid being hit by an errant shot? Not a good rule. Come to think of it, I can't remember ever seeing this sign, or this rule in print on a score card. Probably just missed it.