“then the commerce clause would allow the feds to do it”
Nope, it’s the other way around. The commerce clause limits the federal government’s authority to only regulating interstate commerce, so this mandate could be seen as excessively meddling into private business operations inside the states. Even the Obamacare mandate which was less meddling than this was ruled as a violation of the commerce clause, of course Roberts then got cute and ruled that the penalty could still be enforced as a tax.
EXCELLENT POINT JUSTICE ALITO