At one time the DEA said unofficially that they wouldn't be going after medical marijuana users and growers in California even though they had the legal right to do so because ... well, because they recognized there was a state law and there were more important things to do, and they didn't relish the lousy PR of busting sick people. Traditionally the DEA didn't mess around with grows of fewer than about 1,000 plants -- they're doing their photo-op "eradication" campaign just now -- leaving smaller stuff to state and local authorities -- who under California law are obliged to respect medical users, growers and dispensaries.
However, here's the kind of nasty hardball the feds are starting to play in Los Angeles County. They have notified the landlords of at least 30 different dispensaries that their property is being used for illegal activities and under federal law it could be seized -- stolen -- from them. Most of the dispenary operators don't own their buildings, they rent facilities. Many of the notified landlords have notified the operators that they'll have to close.
This may be only an intimidation tactic -- so far they haven't followed through on anybody yet. It's a particularly nasty one, and it reveals the essential nastiness of drug warriors. They simply relish the idea that they can bully people by threatening to steal their property from them -- all for the sake of denying valuable medicine to sick people. The feds are the really sick ones.