Federal law is enforced by the federal government, not state governments. If the DEA want's to arrest someone for smoking a joint then they have the authority to do so and California cannot interfere with them.
That is an interesting comment.
I don't like analogies, but I'll make an exception in this case.
Suppose one of the former slaves states - Massachusetts, for example - decided today to enshrine slavery into their state constitution. And they set up state agencies to license slave sellers and collect a tax on slaves that were sold. And slaves are actually bought and sold.
(And like California does with marijuana, Massachusetts adopts safeguards to ensure the practice of slavery is not abused.)
Comes now sister DD and says Massachusetts is not defying federal laws against slavery.