Folk smoke, because they fancy trying it, like it, and want to continue enjoying it. Just like numerous other things, alcohol, unhealthy foods, various currently 'illegal drugs' etc....
So what if it kills, everything kills eventually. Tobacco has been made the bogeyman by various governments, based on half-truths and politically motivated spin, there are other arguably equally 'nasties' out there that are swept under the carpet for 'reasons'.
Ingesting smoke, whatever kind of smoke it is, is inevitably going to have a negative effect on people who do it. Yes, it triggers cancer in some, but not in all smokers by a long shot, and even with those it does it typically takes years to occur - At the end of the day it boils down to personal choice, does an individual wish to take the risk of dying from cancer 10, 20 or whatever years down the line, having enjoyed smoking as much as they wished to smoke during that time, or do they wish to deny themselves that pleasure in the hope they live to be 80 or 90. And that is just a hope, not a certainty, as cancer comes along without smoking or doing any of the other 'nasties' too.
It must be a bit galling for someone who tried tobacco and liked it as a teen etc, but denied themselves it in the belief that they would be putting themselves in an early grave, gets diagnosed in their 20's or 30's with terminal cancer anyway. Especially when several of their peers who've smoked like chimneys since their early teens are still running around fit and healthy.
Bans on easily obtained products never work, if prohibition didn't prove that point, the total ineffectiveness of the current ban on so-called 'illegal drugs' should. In addition many bans are an infringement of an individual's freedom of choice, I never asked any Government to decide 'what was best for me', I have never been asked if I wanted a Government to decide 'what was best for me', and they needn't bother, as they'd be told where to go. I object strongly to any Government assuming they have the right to decide 'what is best for me' - I'm happy, and I believe more than capable of doing that for myself, thankyou very much.
What I do with my body which does not directy affect anyone else is nobody's business but my own, and I alone will decide 'what is best for me'. Regardless of whether the 'risks' of 'passive smoking' are real or theoretical, the current ban on smoking in enclosed public spaces is fair enough, other folk have the right to breathe as clean air as is practical even if the reason is only personal preference of aroma, but thats as far as any ban can be justified.
Dictating what folk do in private is opening up a whole other can of worms. If there's a 'health' argument to be made to support a tobacco ban, there's an equally good one to be made for an alcohol ban, or at least the imposition of very strict controls on daily consumption, the same goes for many 'foods' and likewise, participating in sex is as likely to give you a whole plethora of diseases some of which are as equaly horrendous for the sufferer as any cancer. Do we ban alcohol, at least half the stock of your average grocery shop, and all conjugal activities too on 'health' grounds.
We're living in 1984 for too much as is, leave folk to do what they please to themselves as long as they do it in private, its their bodies and lives after all, and nobody else's business unless help is actively sought by them. Living as long as one possibly can is not compulsory, what gives any one person the right to judge that just because someone decides they want everything now, and fills their body with every hellery going that puts them in their box before they're 30 is somehow 'wrong' in their choices. They did what they did for the reasons they did it, just like the one who abstained from every nasty all their life but still died riddled with cancer in their 60's did what they did for the reasons they did it.
Mutual respect for others freedom of choice and personal responsibility for ones actions is rapidly being lost in some vague theoretical politically led societal construct of everybody having a right to interfere and dictate in others personal and private lives, its time that stopped.
If someone wants to sit in private and chain smoke until they're entirely kippered, if someone wants to sit and drink all day every day until they turn their liver and brain to soup, if someone wants to bareback every AIDS carrier they can find until they look worse than someone in The Walking Dead, let them all get on with it, its their choice to make, it directly affects nobody but themselves and is nothing to do with anyone else.