Why did the Anti-Injunction Act matter in the Affordable Care Act cases?

The Anti-Injunction Act was a significant preliminary issue in the Affordable Care Act cases because it generally prohibits federal courts from hearing lawsuits intended to restrain the assessment or collection of any tax before it has taken effect.

If the Supreme Court had determined that the individual mandate constituted a "tax" under this act, the court would have been barred from hearing the legal challenges to the mandate at that time. However, the court ruled unanimously (9–0) that the act did not prevent them from considering the challenge. The court reasoned that because the Patient Protection and Affordable Care Act labeled the payment required for failing to obtain insurance a "penalty" rather than a "tax," the Anti-Injunction Act did not apply to the suits. This allowed the court to proceed to the merits of the constitutional challenges regarding the mandate.12