Lot's of people make those kinds of arguments and they're wrong too.
Just because it is "natural" doesn't mean it's safe or harmless. Tobacco is natural and it grows from the ground just like the curly weed. Both are bad for your lungs and your body, but due to the vastly larger numbers of tobacco addicts we know more about the harmful effects of tobacco. Peter Tosh will tell you weed is safe and even good for you, but he isn't a health researcher.
Opium is also natural as is uranium, snake venoms, puffer fish toxins etc.
I wish we didn't have to deal with the evils alcohol and tobacco either, but using weed makes you a defacto criminal. And while I'll agree that pot isn't as addictive as tobacco, there certain has been a lot of people that have thrown their lives away using it.