George McMahon is not allowed to visit the field where the federal government grows his marijuana. He has stood outside the double razor-wire fence circling the plot in Mississippi, but he's not allowed inside. "I'm not really sure that I want to see it. The thought of all of that medicine locked up, while so many people are hurting, makes me angry," McMahon writes in his recently released book, "Prescription Pot." McMahon suffers from a debilitating genetic disorder called Nail Patella Syndrome. Since 1990, the government has supplied him with 300 tightly rolled joints each month. Only four other Americans...