The Supreme Court's recent ruling in the alcohol-regulation case Tennessee Wine & Spirits Retailers Association v. Thomas corrects an 80-year-old jurisprudential error that raised the price of alcohol for generations. A misunderstanding of the 21st Amendment, which repealed Prohibition while granting states power to regulate the "transportation or importation" of alcohol, had allowed protectionist state laws to interfere with interstate commerce. In striking down Tennessee's in-state-residency requirements for retail liquor licenses, the Court held that the Volunteer State can't discriminate against outsiders willing to set up shop within its borders.

Business licenses have nothing to do with whether a state imports liquor, which is why, as the Supreme Court clarified, protectionist regulations are no more valid in the alcohol market than they would be for any other market. Now that the justices have fixed that derogation of constitutional structure, it's time for Congress to learn from its misreading of regulatory power over alcohol to ensure that state protectionism doesn't replace federal prohibition of another drug, cannabis.