No, the Feds have waived their immunity. The Federal Tort Claims Act, 28 U.S. Code § 2674, permits private parties to sue the United States in Federal Court for most torts committed by persons acting on behalf of the United States. "The United States shall be liable, respecting the provisions of this title relating to tort claims, in the same manner and to the same extent as a private individual under like circumstances, but shall not be liable for interest prior to judgment or for punitive damages."