You're forgetting that government is this country provides a public health care system. Allowing it's citizens to knowingly poison themselves just puts undue pressure on this system.
Smokers are selfish. That's the problem.
The government itself introduced a state funded healthcare system. Thus the problem you allude to was introduced by the state not the smokers. If there was no public healthcare you wouldn't have a case. This is a perfect illustration of why libertarians hate government intervention as it inevitable necessitates MORE intervention to solve problems caused by intervention in the first place.
Secondly smokers pay incredibly high taxes on their cigarettes and tabacco and many pay VAT and normal tax. So they are entitled to benefit from the healthcare system.
Thirdly, if I use your rational and apply it consistently, this would open the door for more intervention in our behaviour that also results in increased burdens on the state.
How about the state forcing everyone to eat a paleo diet for one thing? This is incredibly expensive and will make all grain derived foods, which are cheap and a staple of most people's diet, illegal to consume. More and more studies and evidence have shown that grains are not healthy for humans in general.
The state could also limit alcohol and coffee intake (A little bit of both is more beneficial than not consuming any) but it could also force people to consume a little. Studies have shown that people who consume small/moderate amounts of alcohol generally, statistically live longer than people who don't. What does this mean for muslims? Are they entitled to use their religion as an excuse to put extra burden on public healthcare?
The state could in theory enforce certain exercise regimes, in fact they already do this in public schools in "Life Orientation". I know in my old high school, when my younger brother was still there, they had to run around the field and were awarded marks based on their time taken to complete a lap. Of course unfit people didn't do to well.
In fact people with bad genes are also predisposed to place massive strain on the public health system, relative to people who are "average". Why should the average joe pay for someone else's misfortune? Is that right?
But economically I want to leave you with this lesson when understanding the MASSIVE difference between provisioning of public/private services. If you kept going back to a private service provider for more, they normally are able to provide discounts and charge less (for being a good customer). When it comes to public services, the more you use, the more the state needs to charge you because the "subsidised" service leads to overuse, eventually to caps and eventually to shortages.
In short, you cannot use public healthcare as an excuse.
Again, it comes down to private property and voluntary association.
If you really want to stand by your opinion on the public healthcare costs issue, you should argue for people to opt out in paying a certain % of taxes (based on healthcare of total state budget) and ask for no taxes on cigarettes, also requiring that if someone opts out they may not make use of public healthcare. That way they don't fund your lifestyle choices, and you don't fund theirs.
But that isn't your argument and it certainly isn't the states argument. They want all the taxes from smokers they can get their greedy little paws on.