David Cole in The Nation:
What led Roberts to cast his lot with the law’s supporters? The argument that the taxing power supported the individual mandate was a strong one. The mandate provides that those who can afford to buy healthcare insurance must do so, but the only consequence of not doing so is the payment of a tax penalty. The Constitution gives Congress broad power to raise taxes “for the general welfare,” which means Congress need not point to some other enumerated power to justify a tax. (By contrast, if Congress seeks to regulate conduct by imposing criminal or civil sanctions, it must point to one of the Constitution’s affirmative grants of power—such as the Commerce Clause, the immigration power, or the power to raise and regulate the military.)
The law’s challengers—and the Court’s dissenters—rejected the characterization of the law as a tax. They noted that it was labeled a “penalty,” not a tax; that it was designed to encourage people to buy health insurance, not to raise revenue; and that Obama himself had rejected claims that the law was a tax when it was being considered by Congress. But Roberts said the question is a functional one, not a matter of labels. Because the law in fact would raise revenue, imposed no sanction other than a tax and was calculated and collected by the IRS as part of the income tax, the Court treated it as a tax and upheld the law.