That's because the lethal dose of THC is so large you'd have to smoke 240 joints in a row to get there.
But no claim is too ridiculous for a Drug Warrior to make with a straight face.
Actually, the LD for THC is so large that you couldn't smoke enough to die - it isn't physically possible. You would basically fall asleep (or pass out) long before you could possibly smoke enough to kill you. If you extracted it and concentrated it, then you could get a lethal dose.
As for people who say, "Well, the war on murder, robbery, rape, etc., haven't eradicated those crimes, so we should just stop fighting them too", is making a completely bogus straw-man argument. The difference between marijuana (and I might add alcohol) use and all those other crimes, is that all of those other crimes involve an UNWILLING VICTIM. I would also add that in my entire life, I have never seen anyone who was high on marijuana get into a fight, beat their wife/girlfriend, be loud, rude, and obnoxious, or "smoke" themselves to death. The same cannot be said of alcohol.
Furthermore, one of the reasons marijuana is so vilified is that it is supposedly a "gateway" drug, and its use will lead people to harder, more addictive, and more dangerous drugs. There is precious little evidence to support this claim. There are millions and millions of Baby Boomers out there who smoked pot for many years of their lives (and many, many of them still do, I am sure). In the overwhelming majority of those cases, they never did any harder drugs like cocaine or heroin, though some undoubtedly did experiment with LSD, Mescaline, Psiloscybin, and other hallucinogens.
Is marijuana consumption GOOD for you? No, it isn't, but then neither is alcohol consumption. But you have to put this in perspective: marijuana is NOT a narcotic like cocaine, heroin, methampethamine, etc. It is also NOT addictive like those other drugs. Habit forming? To a degree, but as a (arguably very mild) hallucinogen, it is not an addictive drug (I won't waste space explaining why that is).
People need to get past the knee-jerk "ALL DRUGS ARE EVIL AND MUST BE BANNED AT ALL COSTS" attitude. They are no better than Carrie Nation, whose crusade led to a failed experiment in government intervention that led to gangsterism, corruption, and an enormous and entirely avoidable waste of resources and human lives.
It doesn't make sense to have a prohibition against marijuana any more than it made sense (or was feasible) to have a prohibition against alcohol. And as evidenced by changes in state laws over the past 15+ years, legalization in some form is coming, and it is going to happen. When, and to what degree, and how much the government will seek to control (and tax) it is the only question.