The idea that federal and state law could oppose each other like this, seems odd.
I would have expected a more distinct separation of jurisdiction.
For example, here in Canada, BC has been ready to legalize for a long time. But drug laws are federal, so it has not happened. Now that the feds are planing to legalize it, it also means that even Conservative provinces will become legal.
Whether it should be federal or state/province is debatable, but there's no reason that it should be both.
It is important to remember that the United States are... states. More similar to the EU than to Canada, I think. A great deal of Congress' authority in such matters comes from abusing their Constitutional right to manage inter-state commerce.
What a tool.