Is it illegal for journalists to publish tax information received unsolicited? Federal law says yes.
But that federal law is almost certainly unconstitutional.
The Supreme Court said in Bartnicki v. Vopper (2001) that journalists are free to publish truthful information on matters of public concern as long as they did nothing illegal in obtaining the information. In Bartnicki, a Pennsylvania radio station obtained a recording of a cellphone conversation. Making the recording without both party's consent was illegal. But the recording had been given to the radio station unsolicited, and its contents (about labor negotiations between the city and its union) were truthful and newsworthy.
The radio station was free to broadcast the recording, Justice John Paul Stevens wrote, because "A stranger's illegal conduct does not suffice to remove the First Amendment shield from speech about a matter of public concern".