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.

Here in Colorado pot is sort of quasi-legal medically. A Dr. recommendation (not prescription as you pointed out) is required. However, Pot is not classified as a standard prescription drug, which makes it taxable. The Obama administration has issued guidance about not prosecuting folks who are in adherence with their states marijuana laws, which has greatly contributed to a boom industry in pot sales. Although the Feds haven't always lived up to the pledge as a few folks have been arrested for medical marijuana growth here.

All in all, however the Colorado system is very lax. The fact that it's now taxed gives it additional validation. It's very easy to get a card and dispensaries are abundant (I have several within walking distance of my house in central Denver).