Yes, everyone should get access to healthcare, and since it's currently mandated that everyone must have health insurance, then yes, insurance companies get to pay for it. Not exactly crying a river if their profit margin suffers. I'm looking forward to the day when we go true single payer and they get broken by power of collective bargaining.
Life is not full of black and white issues and to me this issue has a huge gray area. Smoking is legal in this country and people have the right to buy and smoke cigarettes at will. Insurance companies are no longer allowed to deny coverage based on risk factors, this being one of them. No, they should not be allowed to deny coverage for any reason.
First and foremost, Cigarettes have a warning in the pack cautioning against using it as it causes lung cancer and such. Hence. It is fool hardy for insurance companies to be compelled to pay for treatment of such seeing as this is a grave dug by the partaker. It is like suggesting that insurance companies should pay for write off cars caused by drunk driving.
No, insurance companies should not be required to pay for treatment for heart disease, when the patient participates in risky behavior, like smoking, because the person should not be able to collect for a disease that they can prevent. The person needs to take responsibility for themselves and their own health.
I don't believe that insurance companies should be on the hook for people that bring problems on themselves by engaging in activities as damaging to their health as smoking. Insurance is supposed to spread out the risk pool over a large group, but if certain people are deliberately doing something that's likely to cost them more, that's unfair.