I have been wondering about this. It's actually a legitimate argument if they are picking up the tab. If you had to pay for it yourself, you might change your habits.
As a taxpayer, if the government ever became the insurer, I would resent seeing people smoking, etc. then expecting the government (i.e. me the taxpayer) to pick up the tab for illnesses they caused to themselves.
You're already paying the tab for the Medicare and Medicaid participants who haven't done squat to maintain their own health.
The government already does with medicare, medicaid and Tri-care etc.
I would resent them driving, skiing, riding a motorcycle, over-eating, climbing mountains, playing any kind of sports, BBQing, blah, blah, blah. Life is full of risks and people get sick and need care, they get injured and need care. I know so many people who have cancer or died of cancer and so few of them smoked...and those I've known who did smoke had small-cell lung cancer and were dead in months, thereby saving lots of bucks.