“„[T]he norm in America is for the states to decide whether particular behaviors should be made criminal. To make the smoking of marijuana, whether for medical purposes or not, one of those extremely rare instances of federal crime - literally, to make a ‘federal case’ out of it - is wholly disproportionate to the activity involved. We do not have federal criminal prohibitions against drinking alcoholic beverages, and there are generally no criminal penalties for the use of tobacco at the state and federal levels for adults. There is no rational argument for treating marijuana so differently from these other substances.