As Andrew Jackson said of the Supreme Court, "John Marshall has made his decision.  Now let him enforce it."  Yes, the DEA could, in theory, enforce the federal prohibition of marijuana.  But it doesn't want to.  It doesn't mind looking for harmless ditchweed to spray--$100 million or so per year to eradicate harmless weeds--but it doesn't want to be in the business of policing individual possession.  If it did, you'd have seen the Colorado and Washington cannabis industries shut down already.  Now they have Alaska, Oregon and California in addition.  More states coming.  That'd be a great way to blacken the already discredited war on drugs even more--override the states, break up profitable private industry, cut off public revenue, and start shipping teenagers off to jail in large numbers.

Incidentally, I dont think state or local officials take an oath to enforce federal laws.