Tobacco is far too cemented in culture, and there is a key difference between tobacco and weed or heroin.
Cigarettes have a very mild narcotic effect, I mean yes it is frighteningly addictive but it doesn't cause you to trip balls, lose motor function, act out in a psychotic rage etc.
Cannabis may be less physiologically damaging than tobacco (well lung wise anyway), but when you get high, you lose a significant level of alertness, THC is classified as a hallucinogen, and even though it isn't exactly LSD, it causes a warped perception of reality and a loss of motor/mental function. Not to mention people may possibly go psychotic on it, and it can cause schizophrenia in those prone if used chronically. Cigarettes just give you cancer if used chronically, which don't make you an immediate threat to society.
Booze does obviously cause many more bad mental effects, but there are a **** ton of restrictions on who can drink, what they can do under the influence, so basically they're trying every possible way to control a problem that basically can't be stopped with complete outlawing.
Cannabis has never been a universally acceptable recreational substance, and so basically it's not so big a deal to keep it outlawed. People who say cannabis isn't dangerous are naïve and are basically only interested in making it legal so they can blaze in peace.
I mean there's nothing wrong with wanting it legalised, but championing some bull**** notion that it is safe as an ice cream cone is just absurd, and shouldn't be done.