For the record DOJ is directly under the President's control. You can't blame Republicans for this one. If we can't trust Obama to take rational positions on such clear cut issues, who can we trust?
The President is head of the Executive branch. It's his duty to enforce Congress's laws.
Pot is illegal at the federal level (aka Congressional law). It's against the law to possess, sell, etc. So unfortunately the President is legally required to prosecute this.
Now, given that, I am a bit disappointed this is happening. There are plenty of other more important things to prosecute more zealously. Especially given Obama's "bigger fish to fry" comment.
Responsibility to enforce the laws of congress:
http://en.wikipedia.org/wiki/Article_Two_of_the_United_State...
Right to refuse to enforce un-constitutional provisions: http://www.justice.gov/olc/nonexcut.htm
History of the commerce clause (Or "FDR, how we miss your court packing"): http://en.wikipedia.org/wiki/Commerce_Clause
...except when he decides not to defend a law[1] that he claims is unconstitutional.
To be clear, I do actually support universal health care. The Obama administration, however, made a deal with the pharmaceutical company to win their support for the law, one that involved increased crackdowns on medical marijuana and illegal drug imports, and various other moves that benefited the pharmaceutical industry.
So you have Bushite US Attys, and an AG (and President) who have been hostile to State sanctioned MJ.