The War on Drugs is predicated on the all-pervasive powers of the misreading of the Commerce Clause as per Wickard, the horrid decision of Raich which asserted that marijuana grown inside a state never having been sold at all was under the jurisdiction of the interstate commerce clause was only a small hop but still illustrates the length to which the courts will stretch to validate/justify the unjustifiable: not-commerce suddenly is commerce.
The "war on drugs" doesn't go far enough. We should be invading countries that make drugs for sale on US streets. We should be shooting people who run meth labs on the spot. Tens of thousands of americans have had their lives ruined by drugs, their kids lives, and so on. It's hard to believe that Americans do not take this personally, as they should. Americans should respond to drug dealing and drug manufacturing with maximum prejudice.
Libertarian talking point. "Commerce" is cited because it is convenient. The court's gave broad powers in their Wickard decision, so "why not use them" is the thinking of various legal officials. Why would they want to fight the issue out for another justification when they've already got "Wickard"? Why work for something when you don't have to?
A more rational authorization for drug interdiction is in the section responsible for defending the nation. Drugs are no different than chemical or biological weapons, and therefore the constitution authorizes the government to stop them.