New York Times:  “Marijuana remains illegal under federal law, but that has not stopped a fuzzy industry of marijuana farms and dispensaries from rising to serve the 15 states that allow the drug to be used for medical purposes. Under President Obama, the federal government had seemed to make a point of paying little attention — until now. . . . federal prosecutors are suddenly asserting themselves, authorizing raids and sending strongly worded letters that have cast new uncertainty on an issue that has long brimmed with tension between federal and state law.”