I can say the same thing about those having backyard firepits [1] whose output affects FAR more people than a single (e)cigarette. To my knowledge no one is campaigning against big campfire though.
Edit:
> These users often say "it's only water vapor", which is absurdly false.
Yes, but does second-hand e-cigarette smoke cause significant harm? For example second-hand cigarette smoke increases the chance of lung cancer by 20% when living with a smoker [2]. Contrast that with inhaling a puff of e-cigarette second-hand-smoke in a public place once in a while.
I understand that you may find e-cigarette smoke annoying but that's not a sufficient reason to ban something in a free society.
[1] http://www.ehhi.org/woodsmoke/health_effects.shtml
[2] http://www.surgeongeneral.gov/library/reports/secondhandsmok...