their "legitimate" use is that people like vaping, the way people used to like smoking, and people should be allowed to do what they want as long as it doesn't harm others. Smoking was increasingly outlawed because it was found that it did harm others - "secondary smoking" meant unwilling non-smokers were being made ill too; it was to protect them, not the smokers.
If there's any evidence that secondary vaping is a problem they could do the same, but I don't know of any.