From SCOTUS Blog:
Although the Court had four questions before it, the focus of the challenge to the Affordable Care Act (ACA) was the so-called individual mandate – the requirement that almost all Americans buy health insurance by 2014 or pay a penalty. Defending the constitutionality of the mandate, the government’s primary argument was that Congress can require everyone to buy health insurance using its power under the Commerce Clause of the Constitution, because the failure to buy insurance shifts the costs of health care for the uninsured to health care providers, insurance companies, and everyone who does have health insurance. Five Justices – the Chief Justice and Justices Kennedy, Scalia, Thomas, and Alito – all rejected that argument. But the government still won, because a different set of five Justices – the Chief Justice, and Justices Ginsburg, Breyer, Sotomayor, and Kagan – agreed that the mandate was constitutional, but for a different reason.
The most important part of the Court’s opinion on the mandate came from the Chief Justice, John Roberts. He acknowledged that Congress has a broad power under the Commerce Clause, but he emphasized that Congress’s power to regulate commerce assumes that there is commercial activity to regulate. In his view, the mandate creates activity, rather than regulating it. If the Court were to interpret the Commerce Clause the way that the government does, he contended, it would allow Congress to regulate all kinds of new things – including forcing people to buy vegetables (with no specific reference to broccoli, however). “That is not the country” the Founding Fathers envisioned, he proclaimed.
Although the Chief Justice rejected the government’s Commerce Clause argument, he agreed with one of the government’s alternative arguments: the mandate imposes a tax on people who do not buy health insurance, and that tax is something that Congress can impose using its constitutional taxing power. He acknowledged that the mandate (and its accompanying penalty) is primarily intended to get people to buy insurance, rather than to raise money, but it is, he explained, still a tax. If someone who does not want to buy health insurance is willing to pay the tax, that’s the end of the matter; the government cannot do anything else.
Justice Ginsburg (joined by Justices Breyer, Sotomayor, and Kagan) agreed with the Chief Justice’s bottom line – that the mandate is constitutional under Congress’s ability to tax – even while disagreeing with his Commerce Clause conclusion; those four Justices would have held that Congress could use its power to regulate commerce to pass the mandate. With five votes to uphold the mandate, it will survive, and the Court did not need to consider the “severability” issue — that is, what other parts of the law would have to go if the mandate were unconstitutional.
via The mandate is constitutional: In Plain English : SCOTUSblog.