Because some people would still smoke in restaurants and airplanes if left to their own devices.

Technical and legal norms are implementation of social norms. “obviously everybody knows that you are supposed to do it that way” doesnt hold up well at court, or during an argument about any decision that costs big bucks.

Wasn't there a "standard" that phones in south kora had to make a shutter noise when you took a picture because they had such a huge problem with men doing up the skirt photos of strangers?

Not sure why you think social norms are positive. Negative amoral behaviors can become norms. I'd argue the west has tons of amoral behavior that is normalized.

Why do we need laws against stealing bikes?

(Yes, example chosen for its reputation for indifferent enforcement.)