The largest study ever conducted has concluded that pot smoking, even constant pot smoking, does not increase your risk of lung cancer. "We hypothesized that there would be a positive association between marijuana use and lung cancer, said Donald Tashkin, a UCLA pulmonologist involved in the study, "and that the association would be more positive with heavier use. What we found instead was no association at all, and even a suggestion of some protective effect."
Yes, a protective effect. It turns out that THC, the active component in pot, might encourage healthy cell death, knocking off the aged and deformed cells that can trip into the mutating feedback cycle that creates cancer. And just to be sure the study was correctly carried out, they also asked respondents about cigarette smoking, finding that two packs a day created a 20-fold increase in lung cancer.
Quick thought experiment: if a martian landed on this planet, and you explained to him that there were these two types of cigarettes, one that was violently addictive and increased your susceptibility to a deadly disease by 2000%, and another that was pleasant, non-addictive, and protective against the killer ailment, which do you think our friendly neighborhood martian would theorize we keep legal?