On the surface yes but when a ballot initiative is in direct conflict with federal law the constitution gives supremacy to the Feds. That is exactly why the various pot laws have gone nowhere because no state government will defend a law passed outside the legislature.
Like Davis saying Cali has the right to impose higher pollution standards than the EPA calls for. He doesn't admit it straight out, but he's making a "States Rights" argument and defying the FED.
I also think he's right. (I can't believe I said that..)