Ginseng root has been used for 5,000 years in Chinese medicine. Possibly because we're too cautious. As such, it may make no outright preventative or curative claims, but labeling may indicate uses and effects on bodily functions. Drugs must undergo double-blind, randomized testing. Drugs must provide documentation about use, safety, ingredients, indications, warnings and dosage.
Herbal supplements require no such regulation.
In a paper he recently presented to the National Institute of Health's new Office of Alternative Medicine, herb honcho Rob McCaleb theorized that the demand for strict scientific regulation of herbal supplements was hindering access to traditional healing techniques. The inventors of Prozac probably wouldn't mind that kind of investment, given the juicy patent waiting at the end of the laboratory trail. Nobody gets a payoff. He who patents, prospers. Because no one can patent an herb, no one can exclusively profit from it.