That's an interesting point. I can't believe there is not a way around that though. Congress passed the Controlled Substance Act in the first place. They could modify it. They still add drugs for scheduling. Seems like they could take one away. I do not believe alcohol or tobacco are scheduled drugs. And I'm not sure about this but I think Congress has voted to actually reschedule drugs in the past as well, or maybe all they did was pass resolutions “authorizing” the DEA to look at a drug fro rescheduling like they did with Rohypnol. I don't know enough about the process but I think Congress could legalize marijuana if they wanted to, one way or another, subject to the President's veto power. Congress set up the schedules initially and delegated the power to add new drugs and reschedule those already scheduled. Seems like they could “undelegate” the power for one or more drugs. Personally, I don't think it should be a scheduled drug. I think we should just regulate it similar to the way we regulate alcohol.
I believe alcohol and tobacco were specifically excluded as part of the legislation, but other than that it was pretty open ended in terms of the DEA's authority to schedule everything else. The CSA was passed amid a lot of controversy about mj, and was prompted by the USSC finding the enforcement provisions of the Marijuana Tax Act unconstitutional. I believe they passed the CSA with marijuana provisionally a Schedule I drug, authorized the Schafer Commission to research and make recommendations on scheduling, and left it to the DEA to determine what the final scheduling determination would be.
Since then, Congress has also passed legislation committing us to abide by provisions of UN anti-drug treaties, and the mj provisions of those seem to have been written to conform to DEA policy - mj is not legal for anything other than strictly controlled research. Previous attempts to get it's status changed have resulted in Congress saying "Go talk to the DEA, they have the authority to change the scheduling.", and the DEA saying "Go talk to Congress, they passed this treaty and our hands are tied."