Their bodies had adapted to nicotine and since they chose to quit, they did. Those people simply choose to continue smoking, even if a doctor or loved one has suggested they quit. They aren't suffering from a weak will. And ironically, they are often the ones who transform their iron will into an iron fist, demanding they be financially compensated for the consequences of their own behavior. There's nothing particularly unusual about noncompliance with medical advice or blaming others for one's own behavior.
Many people continue to engage in certain behaviors against medical advice.
Smoking and quitting, like eating and dieting or exercising and being a couch potato, are matters of free will and personal choice. Cancer is a disease. Smoking is a habitual behavior. Moreover, likening a behavior to a disease seems especially cruel to people with real diseases. A person cannot choose to quit or moderate diabetes.