The Coca plant is illegal in the USA unless you have a special* license from the DEA. If you were to have the plant in this country, the land of the free would lock you up. You would be better off killing someone in the USA than have a coca plant.
As far as Hemp is concerned, you are right about the low levels of THC, but there are "levels" of it. THC at any level is illegal in the USA, federally at least. Hemp contains very, very little THC but, non the less, it does contain it. So if it be called illegal, then it should be illegal for all.