I think people here are getting hung up on having actual proof of being a unlawful user. They don't need a blood test to determine that.
This case has made it through two court challenges already. I highly doubt SCOTUS picks it up and I feel the drug will be rescheduled before they do anyway.
Did you read my link? I would think that an equal protection under the law clause could be made. The fact is, there are and have been legal MJ users under federal law.