People indulge constantly in things that are unhealthy for us. It’s basically our favorite shit.
Within reason and as long as they’re not hurting anybody, try not to judge too much. Everyone has different lived experiences and find their own ways to cope.
As for movies… if you’re shooting a historical film of any kind, it would be silly to pretend people weren’t smoking then. There are also characters in every genre that are just written as smokers. We’re human and imperfect, and stories include imperfect people.
As for the actors themselves… they use some kind of herb cigarette that doesn’t include nicotine. Actors can usually hold the smoke in their mouths and not inhale, which makes them much less harmful.
And even if none of that were true and they’re just lighting up real cigarettes, money. Actors get paid, and unless the actor is smoking in every scene for months on end, it’s unlikely to amount to any long term damage.