Feds cracking down on California medical marijuana dispensaries - latimes.com
It's not clear to me the reason for this crackdown. The Obama administration had previously declined to prosecute medical marijuana in states where it was legal under state law.
The legal justification for the crackdown is that marijuana possession is illegal under federal law. The constitutionality of the law follows from the commerce clause, (Gonzales v. Raich - Wikipedia, the free encyclopedia) according to most Supreme Court justices, including Scalia.
That being said, even though the federal government has the power to go after medical marijuana (which in practice it clearly does) I don't think it should. There doesn't seem to be any compelling reason to, as far as I can see.
The bastardization of the commerce clause
http://www.usmessageboard.com/politics/173469-the-bastardization-of-the-commerce-clause.html