The Supreme Court unanimously held that the Anti-Injunction Act does not bar pre-enforcement challenges to regulatory requirements that are not themselves taxes, but are enforced through tax penalties. The Anti-Injunction Act narrowly, but powerfully, forbids lawsuits undertaken for the purpose of restraining the assessment or collection of any federal tax. Instead, a taxpayer must pay the tax and then seek a refund. For tax penalties, that would mean deliberately violating a regulatory requirement and then arguing, in a refund action, that the requirement was invalid. Even though some courts had construed the Anti-Injunction Act to sweep that broadly, the Supreme Court did not.
The U.S. Chamber filed amicus briefs at the cert. and merits stages in support of this outcome. The case is CIC Services LLC v. Internal Revenue Service.