It is my understanding that a lot weed users seem to think that it is not only harmless, but healthy.
Because it's "natural," it can't possibly be bad for you. Not like, say, aspirin, which is just yucky chemicals.
Interestingly, "it's natural" also means smoking weed should be legal, and also God doesn't mind if you're stoned all day. I wouldn't be surprised if "it's natural" is also a defense for driving while stoned. Seems to be an all-purpose justification.
(Our next topic: breastfeeding is natural, so it's OK to expose yourself to everyone.)