I'll be the first to tell you that medical marijuana is in a better position politically than ever before. Even the recent ugliness in Colorado doesn't approach what we've seen in years past (and you know it's true so don't accuse me of Obama-worship or naivety). But the fact remains that the current federal rules of engagement are impossibly vague and will inevitably become deeply problematic for both sides.   

LA Times explains perfectly why this is so:

The confusion can be resolved only by Washington. Fourteen states currently have medical marijuana laws, and more are likely to adopt them, multiplying the legal disarray exponentially.

The new policy of respecting state laws is already helping to expand the medical marijuana map (NJ, DC), yet the feds still claim the right to intervene at their own discretion. DEA enforcement against clear violations of state law might be tolerated politically, but their active involvement becomes less sustainable as new states enter the picture. As long as DEA maintains its authority to enforce local regulations, any inaction on their part will inevitably resemble tacit approval. It makes far more sense to step aside entirely and let state police and state courts take full responsibility for interpreting and enforcing their own laws.

If the White House wants to shield itself from political fallout over medical marijuana, the quickest and easiest approach is to get out of the game altogether.