Because with public health policy all nuance gets lost. If you say "well actually nicotine doesnt harm you but the tar in combustible cigarettes does so if you vape you're really doing no harm" people will render that as "nicotine does not harm you thus cigarettes are fine" this is literally how we got into this situation. There can be no grey area: if you do not smoke combustible cigarettes do not vape.
Btw all addictive substances are bad for you by definition.
Which makes it insanely hard to change things for the better, normal cancersticks (now without menthol!) will continue to be made and sold at extreme tax rates whereas something that isn't perfect but is better will get banned.
I would suggest that loss of nuance is true in some societies and not others.
We can and should be working to increase the fidelity of communication as well as make policy less of a blunt instrument. Education can work wonders as can honest, real, considerate conversations. All of these things benefit all of us.
Things are different today when compared to our young experiences. Younger people have far more information than anyone making these policy decisions does.
People did make that poor leap. Yes.
Will they do it again? I doubt many of them will, and the ones who might are likely to be using pot and will combust and inhale.
Society has far different norms and can handle nuance far better today. We need to recognize these changes and treat people better.
They can and will respond. They have tech and the body of common knowledge continues beyond what it was two decades ago.
This kind of "we know better for you" communication packs a lackluster punch and may do more harm than good.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4363846/