The Supreme Court ruled on this issue in 1972. It is black letter law that “a reporter called to testify before a grand jury regarding
confidential information enjoys no First Amendment protection.”
Branzburg v. Hayes, 408 U.S. 665 (1972).
The only testimonial privilege for unofficial witnesses that is rooted in the Federal Constitution is the Fifth Amendment privilege against compelled self-incrimination.” Id. at 689-90.
The Court expressly declined “to create another by interpreting the First Amendment to grant newsmen a testimonial privilege that other citizens do not enjoy.” Id. at 690.
The Court also stated that it could not “seriously entertain the notion that the First Amendment protects a newsman’s agreement to conceal the criminal conduct of his source, or evidence thereof, on the theory that it is better to write about a crime than to do something about it.” Id. at 692. “the right to withhold news is not equivalent to a First Amendment exemption from an ordinary duty of all other citizens to furnish relevant information to a grand jury
performing an important public function.” Id. at 697.
Bottom line, the appellate court in Miller's case was convinced that there was reasonable cause to show that Miller (and Cooper) had information about a potential violation of federal law, and saw no reason why a newsman (woman) should have any greater rights than any other citizen.
This is not to say that Miller, Cooper or any reporter has no recourse. They can, if the proof is there, move to quash the subpoena. (See, e.g., "If the newsman is called upon to give information bearing only on a remote and tenuous relationship to the subject investigation, of if he has some other reason to believe that
his testimony implicates confidential source relationships without a legitimate need of law enforcement, he will have access to the court on a motion to quash and an appropriate protective order may be entered. Id. at 710). They probably have the right to sue the prosecutor as well.
You can read the Appellate court decision here:
http://pacer.cadc.uscourts.gov/docs/common/opinions/200502/04-3138a.pdfI really suggest that all who question this decision read it first.
As an (important) aside, the author of the majority decison in this case was David Sentelle, the appellate court justice involved in appointing Ken Starr.
(See, e.g.
http://www.consortiumnews.com/1999/c041799b.html)