Is that true? Which law?

I always thought that Delta's policy against it was just political correctness masquerading as passenger comfort. After all, there are a lot of activities that do not inherently impact others if done courteously, e.g., the aforementioned eating of sunflower seeds, chewing gum, blowing one's nose. Should all of these activities be disallowed on board? No, only chewing tobacco, because smokers (and all tobacco users I guess) are one group that are allowed to be treated as second-class citizens these days.

I have never seen smokeless cigarettes, so don't know the impact on the environment, but Qatar Airways explicitly states that these are not allowed.