Comment by pessimizer

The obvious answer is because the old, crudely drawn sign did (and to go deeper, you'd have to ask the janitor who drew it.) Now it's become completely detached from its origin and pointless.

The only reason this question is answerable is because the poster saw the old sign. When everybody is gone who saw the old sign, they'll just continue doing it for no reason. When somebody asks about it, some creep will babble about Chesterton's Fence.