On certiorari, the United States Supreme Court held that federal whistleblower law protected MacLean from termination.
Federal law prohibits federal agencies from taking personnel actions against employees who disclose information revealing "any violation of any law, rule, or regulation," or "a substantial and specific danger to public health or safety," as long as the disclosure is not "specifically prohibited by law." 5 U. S. C. §2302(b)(8)(A).
The TSA promulgated regulations prohibiting disclosure of "sensitive security information," including the type of information that MacLean disclosed to the media. However, the Court held that these regulations are not "laws" within the meaning of the whistleblower statute. Thus, although prohibited by TSA regulation, MacLean's disclosures were not "prohibited by law," and MacLean was protected from termination.
The Court also held that the federal statute that authorized the TSA to issue the regulations - 49 U. S. C. §114(r)(1) - did not itself prohibit MacLean's disclosures. The Court held that section 114(r)(1) only authorizes the TSA to issue regulations and does not itself prohibit anything.
Chief Justice Roberts wrote the opinion, joined by Justices Scalia, Thomas, Ginsburg, Breyer, Alito, and Kagan. Justices Sotomayor and Kennedy dissented.
The opinion is available here.