This is your best argument, (that it is within the domain of the states to decide) but it does however fall short. Perhaps not with Marijuana, but when you extend the principle behind it to other drugs, an argument emerges for defacto Federal involvement in interdiction.
Regarding such drugs as Opium, Heroin, Cocaine, etc. they generally come from foreign countries. It is beyond the ability of a state to operate in this environment, and therefore it requires a Federal approach to deal with it. Now you may ask, "On what basis can you justify Federal involvement in interdicting drug shipments into the states?"
The Federal government has a mandate to provide for the defense of the country. If drugs are regarded as an existential threat to the security of the nation, (which I argue that they are) then the Federal government has a duty to involve itself in the protection of our nation from this dangerous low-intensity attack. Just as we interdict nerve gas, biological warfare agents, or nuclear fissionable material, so too must we protect the nation against dangerous lesser agents.
Marijuana really doesn't reach the threshold of dangerousness that other drugs constitute, but it is however included in the threat list due to it's classification as a drug. The largest danger it represents is the tendency of others to use it as justification for the legalization of drugs as an acceptable principle.
“The largest danger it represents is the tendency of others to use it as justification for the legalization of drugs as an acceptable principle.”
Well, that’s the fault of the people who classify it as a drug, isn’t it?