The idea, I think, is that humans have been sharing the planet with everything else for so long that anything especially dangerous that occurs "naturally" is likely to have been already tested- ie "artificial" drugs should be more unpredictable, and therefore potentially more dangerous, than whatever stuff some exotic culture has been fucking with for hundreds of generations (which is pretty much any interesting compound you can find "naturally").

Of course, unscrupulous-types will lean on whatever marketing stunt they feel will sell some short-term product, with little respect for the long-term dangers/benefits, so naturally words like "natural" and "organic" get abused.