Putting the burden of proof solely on the journalist leaves the press open to legal harassment, having to constantly prove in court every statement that a public figure doesn't like. That kind of law probably wouldn't pass a constitutional test, especially in front of a Supreme Court that has upheld the right of news organizations to lie on air.
Public figures can't win defamation cases based on truth alone. Public figures have to prove malice to win a defamation case. That is, not only was the statement untrue, but the journalist knew it was untrue, and wrote it with the purpose of defaming the public figure.
