Because freedom can't protect itself
As I see it, if the Executive and Legislative branches pass a law which includes in this law statements which provide immunity from the finding of guilt or innocence by reason of breaking this law, this, de facto, should be unconstitutional as it completely infringes the domain of the Judiciary. Of course, in the case of the FISA law with its immunity provision, this is ex post facto and prohibited by the U.S. Constitution so res ipsa loquitur would apply.
I cannot support a blanket shield law that would protect the media when it disseminates government propaganda from anonymous "high government officials" or when it protects a Scooter Libby from prosecution. When the Fourth Estate is providing the oversight, and exposing wrongdoing, a shield to protect the whistleblowers and their interviewers is very appropriate. It is much less so when it protects the government from scrutiny by the public and the courts.
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