The fact is that cancer is a very complicated situation. It is caused when the genes that control the cell are mutated, either via chemical toxins or by radiation, and then the cell grows out of control. Some viruses are capable of causing these mutations, but most are the result of chemical or radiation damage to the cell nucleus (another reason why the Witlesses are banned from further education).
A person can get cancer from exposure to chemicals. These come from the workplace, the food you eat (and if you eat the fast, good-tasting poison the Witless pioneers eat all the time, you are getting a hefty dose of those chemicals), metabolism, and yes, smoking. Radiation can come from the sun, cosmic flares, and radioactive elements in the environment. The use of sunscreen is hotly debated, since there is evidence that some unprotected sun exposure protects you, and some of the chemicals in the sunscreen may themselves cause cancer. Still, getting too much sun (to the point of near-sunburn) is not a good idea, especially on a regular basis (and still one wonders why skin cancer rates in the tropics are lower than in the temperate regions).
Many cancers can be prevented. Not smoking is the big one here (and this comes from many a mainstream medical source as well as mainstream newspapers). Get plenty of exercise, cleanse the poisons out of your body regularly (or eat fewer of them in the first place), keep your weight down to a sensible level, eat organic foods (especially those where pesticide residue is highest), and avoid excess radiation (while some sun exposure is OK, those tanning beds are not). However, cancer can still hit with all these in place.
Much research is needed in cancer treatment and cure (at that, research that doesn't keep us in the 1950s perpetually). In the meantime, blaming the victim is not really all that sensible in most cases. And that goes especially for when the person quit smoking 30 years ago, or never smoked, and still gets cancer. Of course, if the Witlesses refuse to go along with that, I can point to the number of them that have died of cancer in their 40s and 50s, usually from wretched diets and too much stress and exposure to car fumes in the field misery.