Why did the Supreme Court rule that the Anti-Injunction Act did not prevent review of the individual mandate?

The Supreme Court ruled unanimously that the Anti-Injunction Act did not prevent it from reviewing the individual mandate because the Patient Protection and Affordable Care Act (PPACA) explicitly labels the payment required of those who fail to obtain health insurance as a "penalty" rather than a "tax."1

The Court determined that the mere fact that this penalty is assessed and collected in the same manner as taxes does not require it to be treated as a tax for the purposes of the Anti-Injunction Act. Consequently, the Act—which generally bars lawsuits seeking to restrain the assessment or collection of taxes—did not apply to the challenge against the mandate.1