Both of those positions may be overreaches. It's true that federal marijuana legalization would require revision of the drug treaties, if the US is not to be in violation of them (or for the US to do what Bolivia did by withdrawing and then rejoining "with reservations"). Legalization by Congress even just within states that have enacted it is also a likely treaty issue. But Colorado and Washington aren't parties to the treaties, and federal law remains in force within those states. The states have simply ceased to contribute their own resources to a part of the prohibition program. Under our federal system they very probably have the legal right to do so.
And that is why the DEA chiefs have overreached as well. When one says that federal law is supreme in this area, it means that federal agents can use the powers they have to bring criminal or civil actions against marijuana users or sellers, despite the passage of state laws -- the Raich case decided that for medical marijuana, for reasons that would seem to apply to fully legalized marijuana too. But that doesn't mean the states have to help them. We have a federal system. As I've pointed out previously, no federal prosecutor in 16 years of state medical marijuana laws has ever argued in court that the states can't have those laws on their books. Clearly they've had incentive to do so, if they thought they could win that way.
I don't argue that we know for sure how these points will come out if they are adjudicated -- it is new legal territory. But most legal scholars seem to think a preemption ruling would be a long shot outcome. So that is how it looks to me.
[If you haven't already, please order the two recent reports, from the Cato Institute and the London School of Economics, addressing these two very issues -- available in harcopy on our web site for a small donation.]