It is not. The federal government has simply chosen not to challenge the states on this issue... so far.

Frankly because it would be extremely unpopular. Even in the right wing places, where there is a correlation to less favorable views of marijuana, there's a high correlation to belief that states rights should trump federal rights. Really they believe that the fed should be minimal and things mostly run by the states.

Any examples with regard to "national security" i.e. mass surveillance, perpetual war, besides Kennedy?