The federal laws require that the local jurisdictions deem the sale of marijuana to be unlawful. The federal government depends on local law enforcement agencies to enforce the local laws. The local jurisdictions can choose not to enforce those laws. Local law enforcement has only so many resources, so they can choose to go after the "real" bad guys like murderers and rapists instead driving around looking for drug deals to bust. While I was talking primarily about "black market" sale of marijuana should all the dispensaries close down, I suppose the local governments can choose not to shutdown the dispensaries as well. Obviously, the dispensaries would take the risk that the feds would shut them down themselves, but the local law enforcement doesn't necessarily have to help do so.