The FedGov doesn't give a damn about the medical issues with weed. It's all about MONEY.
The War On some Drugs is HUGE money, both to the drug pigs and the drug pushers. Without the War, prices would plummet, the drug kingpins' empires would collapse and several thousands of JBTs would have to get honest jobs. Gangs based on drugs would have their income source yanked right out from under them.