Is the idea that since smoke can drift over state bounderies, it must be a Federal affair? Other than that it's a bit hard to envisage how it could be anything to do with the rest of the USA if an individual state decides that it's OK using dope.

At least with alcohol they had to bring in a constitutional amendment to make Prohibition binding across the USA. I'd have thought that the logical consequence of that precedent would be that, without such an amendment, enforcing Prohibition of grass cannot be a Federal concern.