In the 2014 TV show Constantine, John Constantine is a cigarette smoker, just like his comic book counterpart. However, we rarely (never?) see Constantine actually smoke (i.e. breathing in and out the cigarette smoke). For instance, he will pull out a cigarette and lighter, but then get distracted before actually smoking and put it away. Even in the crossover with Arrow, we see Constantine stamping out a cigarette, but not actually smoking.
Why is this? Did actor Matt Ryan not want to smoke onscreen? Is there some sort of rule that smoking can't be portrayed on a television show? Why did Constantine go out of its way to never show the titular character smoking?