While I agree saying that weed is "natural" therefore "good" is a bad and weak argument, let me play devil's advocate for a moment.

I think the argument's suggestion is that marijuana, as an illegal drug, differs from other illegal drugs because it does grow naturally in the environment. I think the argument is used to set it apart from meth, crack and all those other evil drugs.

Right on though, it's a fairly week argument and I would never use it to defend marijuana. But I think the argument is meant to illuminate the idea that weed is not like the other illegal drugs, which are usually manufactured in homemade labs.