Well the federal government is certainly pretending state law doesn't exist. They have to be extremely careful when playing this game though. If they go too far, it'll end up a federal case and then the federal government is fucked. That's why they are witting letters to landlords pressuring the landlords to evict rather than just raiding all these clubs themselves and arresting them. While clubs are getting raided by the feds, they are trying to avoid doing the raids themselves in most cases. And they are being extremely picky in which dispensaries they will actually raid themselves.
For example, they raided Richard Lee. But they aren't actually going to take him to court. Richard Lee was doing a lot more than running blue sky and Oaksterdam. I've been to 2 of his "measure Z" dispensaries which are cash only places and not officially approved by the city. It seems like they busted at least one of these measure Z spots. But even when they did, they didn't press charges on him for selling marijuana. They just had a little talk with him and he retired.
If you look at most of the clubs they've actually raided, the charges were almost never for just selling. Nearly all of them were either in violation of some state/local law, had IRS related charges, selling to out of state people, within 1000 feet of a school, or some other technicality. I could be wrong about this, but I don't think any of these raided dispensaries were charged federally only with distribution of marijuana. There was always some other charge involved. In the cases where distribution was the only charge, the feds might have participated in the raids, but they have local law enforcement make the arrest and file the charges.
Basically, Obama and Holder are being EXTREMELY careful about who they bust and what they bust them for. It's a tightrope walk. They are doing everything they can to bust as many dispensaries as possible while making sure the words "medical marijuana" never appear in any case that could potentially reach a federal court. If they make a mistake by going to far, this will all blow up in their faces.