The problem, as I understand it, is that the Feds have the letter-of-the-law final authority to control substances, including deciding which ones are legal. But in practice, it's state, county and city authorities (local cops) that enforce that law. In theory, the federal authorities could enforce federal law related to medical marijuana, but in practice they don't. This is a real problem, since several states have laws related to medical marijuana that contradict federal law.