In a 6-3 opinion delivered by Justice John Paul Stevens, the Court held that the commerce clause gave Congress authority to prohibit the local cultivation and use of marijuana, despite state law to the contrary. Stevens argued that the Court's precedent "firmly established" Congress' commerce clause power to regulate purely local activities that are part of a "class of activities" with a substantial effect on interstate commerce. The majority argued that Congress could ban local marijuana use because it was part of such a "class of activities": the national marijuana market. Local use affected supply and demand in the national marijuana market, making the regulation of intrastate use "essential" to regulating the drug's national market.
And if in a case pitting a state against the feds, we for some reason decide to let one of the parties decide, and the feds find in favor of....the feds, how many states do you think will find that ruling legitimate?