If brains were gunpowder, one is sometimes tempted to think, then the U.S. couldn’t blow its own nose. Even as it sues Big Tobacco, America spends millions each year to subsidize the weed — a plant that ranks, on the Healthful Substances Chart, only slightly above strychnine. Yet it forbids the raising of another crop that presents no threat to health and offers a multitude of uses: industrial hemp.