And, yes, it's true that 'no smoking' often has local law to help drive the idea home, but it's not like there's a cop monitoring the smoking situation in every public venue. Shame, ridicule, side-eye and public opinion are really what keep behavior in line.
It's not hard to do. I remember long ago that the famous Alamo Drafthouse movie theater, which serves beer and booze, had problems with people talking. They started playing a stock reel of people getting their asses kicked overlaid with "if you talk we'll kick your ass out" before the movie started. Now it's just accepted fact that you keep quiet no matter how many Lone Star tallboys you've had.