But "less is more" obviously doesn't work for no-smoking signs

Why do we have to put a "no-smoking" sign on our office door? We apparently risk a £2,000 fine for not doing so.

What is the point of such a sign? I quite understand needing signs when people might otherwise not know the law (e.g. speed limit signs). But not smoking in public places has surely got to be in the top ten of laws that people do (now) know about. And if they don't, they are soon going to find out with or without a four inch sign on our door.

There are all sorts of things people are prohibited by law from doing in our office, including committing fraud, committing murder, driving a car without a seatbelt, trading whilst insolvent, fixing a gas boiler while not competent to do so, etc. but we don't put signs up for those things. Requirement to display a no-smoking sign on every public door (especially if you occupy, say, a gorgeous listed building, which alas we don't) is a bit silly, if you ask me.