The Justice Department’s new policy guidelines concerning medical marijuana, if accurately described in the press accounts represent a positive step toward a more rational drug-control policy and greater respect for state-level policymaking. There are more federal crimes on the books than federal prosecutors can ever hope to prosecute, so it makes sense for the Justice Department to set prosecutorial priorities. The federal government has a legitimate interest in controlling interstate drug trafficking, not in prosecuting those who seek to provide medical marijuana to local residents pursuant to state law. So it only makes sense for the Justice Department to tell federal prosecutors to focus their efforts on those who are not in compliance with state law.
Ideally, the federal government would treat marijuana like alcohol, retaining a federal role in controlling illegal interstate trafficking but leaving each state entirely free to set its own marijuana policy, whether it be prohibition, decriminalization, or somewhere in between. I don’t expect the Obama administration to promote such legislation, but it would represent the proper approach to marijuana use, medical and otherwise.
[Note: I have a longer version of this post on The Volokh Conspiracy.]