The President, Attorney General and all US Attorneys still have to abide by the law. The law in this case is caught up in partisan politics. Obama isn't Stalin bro, he doesn't lay down the law like that.
LOL. I was once that young and naive, too.
First of all, the President AG, and US Attourneys are SUPPOSED to obey and enforce the law.
But we all know damn well that they sometimes ignore or deliberately break the law. Go read about "Operation Fast and Furious" a bit if you think the Obama administration loves obeying the law.
Next, NO US law enforcement agency has the budget to vigorously enforce EVERY law. They'll enforce egregious violations, if they need to, but every enforcement agency SETS PRIORITIES.
The priorities are set at the top by the AG, who acts at the behest of the President. In short, the President sets the tone for his administration in terms of what priorities he wants his LE to pursue.
If Obama doesn't want Federal MJ law enforced, all he has to tell the AG, DEA, etc not to enforce it. He could say, "MJ is low priority for my administration, go concentrate on busting heroin and coke dealers instead", and their response would either be "Yes, Mr. President" or "Here is my resignation".
And in fact that's EXACTLY what he DID do at the beginning of his administration. Recently, he reversed course, and directed the AG to go pursue MMJ dispensaries.
You can speculate about why (obviously "politics", but which politics?), but the recent crackdown on MMJ in CA and elsewhere is the DIRECT result of an explicit change in policy by the Obama administration.
Obama is directly responsible for it, period.