Here is the decision, the concurrences, and the dissent. Justice Gorsuch's concurrence explains what this was about:
Our dormant commerce cases usually prevent States from discriminating between in-state and out-of-state firms. National Bellas Hess, Inc. v. Department of Revenue of Ill., 386 U. S. 753 (1967), and Quill Corp. v. North Dakota, 504 U. S. 298 (1992), do just the opposite. For years they have enforced a judicially created tax break for out-of-state Internet and mail-order firms at the expense of in-state brick-and-mortar rivals. As Justice White recognized 26 years ago, judges have no authority to construct a discriminatory “tax shelter” like this. The Court is right to correct the mistake and I am pleased to join its opinion.
As Justice Kennedy explains, the law includes a number of protections for small businesses:
South Dakota’s tax system includes several features that appear designed to prevent discrimination against or undue burdens upon interstate commerce. First, the Act applies a safe harbor to those who transact only limited business in South Dakota. Second, the Act ensures that no obligation to remit the sales tax may be applied retroactively. Third, South Dakota is one of more than 20 States that have adopted the Streamlined Sales and Use Tax Agreement. This system standardizes taxes to reduce administrative and compliance costs: It requires a single, state level tax administration, uniform definitions of products and services, simplified tax rate structures, and other uniform rules. It also provides sellers access to sales tax administration software paid for by the State. Sellers who choose to use such software are immune from audit liability.
Interestingly, all nine justices agreed that there was no rational basis for the "physical presence" standard of
Quill and that online retailers should no longer be exempt from sales tax. The four dissenters, however, concluded that the Court should leave it to Congress to fix this -- even though the Court had created the problem in the first place.
By the way, Indiana passed legislation substantially identical to South Dakota's. As contemplated by the Act, enforcement of that law has been stayed pending the Supreme Court's decision in
Wayfair. Presumably that stay will soon be coming to an end, and online retailers will start collecting and remitting Indiana's 7 percent tax on sales made to Indiana residents.