Post no Bills

Friends of mine will recognise this rant.. So bear with be while I explain why this simple sign, sprayed all across New York, drives me so irrationally mental.
Post No Bills. We know what it's trying to say right? It means, you are not allowed to put up fly-posters/adverts; bills being the word here for those large bits of paper or flyers. Post no bills. What bothers me, and always has - is the fact it says Post No Bills, not Don't Post Bills. By starting with an action word - "post" - its commanding you to do the action, and what action is that? To put up "no bills". To actually put up "a nothing". The sign commands you to almost go up to the wall and mime putting up a bill - not a real one.
Yes, I know I don't make sense. But nor does the sign. And it's everywhere. Who decided to use such bad grammar??