Why is it Illegal?

What is the point of making it illegal for children to buy cigarettes, if it's okay for them to smoke them? If they're going to be allowed to smoke them, which they should be, why can't they buy them? I mean, it is their life that they are putting in danger, so they should be responsible because if someone buys a child a cigarette and that child dies due to lung cancer or another disease caused by smoking because he or she was allowed this constant supply of cigarettes, then the supplier would be at fault, or at least feel at fault, for the child's death. Then said supplier has to live with the guilt of giving a child cigarettes when that supplier probably knew the harm it could cause.