Andrew, yes that notion that the judge can usurp executive powers when some among the executive branch might have a conflict of interest is the lynchpin. After I read, last night, Torres’s decision demanding that Taricani name his source (PDF), questions about the procedures and powers involved with forming grand juries and appointing special prosecutors still loomed over the issue. (And it isn’t an area of the law that I’m able to research right now.) Taricani’s lawyers’ not making a big deal of that aspect seemed to indicate that Torres hadn’t stretched the law on that count, but I guess they have raised the issue.
Particularly of note, from the Providence Journal piece, is this paragraph:

Taricani, [Torres] said, had no right to refuse to reveal his source to DeSisto, based on the U.S. Supreme Court decision in the 1972 case of Branzburg v. Hayes. In that case, the nation’s highest court rejected the argument that reporters have a First Amendment right to refuse to answer “relevant questions put to them in the course of a grand jury investigation or criminal trial.”

I haven’t read Branzburg, but in Torres’s summary of and quotations from that case in his ruling, the repetition of the phrase “grand jury” is conspicuous. Consider (emphasis added):

Only where news sources themselves are implicated in crime or possess information relevant to the grand jury’s task need they or the reporter be concerned about grand jury subpoenas. Nothing before us indicates that a large number or percentage of all confidential news sources falls into either category and would in any way be deterred by our holding that the Constitution does not, as it never has, exempt the newsman from performing the citizen’s normal duty of appearing and furnishing information relevant to the grand jury’s task.

Again, I’m not versed in the laws and practices surrounding court-appointed special prosecutors, but the difference between them and grand juries strikes me as significant. The sentence that the Projo quotes from Branzburg seems to leave open further possibilities, but it only adds “criminal trial,” which (if I’m not mistaken) suggests a court proceeding in which an indictment has already been made.
Two other cases that Torres cites raise interesting considerations. First, in Bruno & Stillman, the court ruled that, “as a threshold matter, the court should be satisfied that a claim is not frivolous, a pretense for using discovery powers in a fishing expedition.” When Torres turns to another case, Cusumano v. Microsoft Corp., for an example, he highlights the central concern in the Taricani ordeal: in Cusumano, the court was weighing the claims of Cusumano against those of Microsoft; here, the court is weighing Taricani’s claims against… its own.
To be sure, Torres’s strongest point is that the public has a significant interest in maintaining the authority of courts during legal proceedings as well as during investigations undertaken in the course of “law enforcement.” The problem that this neatly sidesteps, however, is that the court is not immune to conflicts or excesses. The party most directly wronged by Taricani’s refusal to answer questions (as quite distinct from the party wronged by the breaking of the protective order in the first place) is the court itself. Even somebody who disagrees with my highly suspicious view of judicial power ought to be able to understand, in this context, why the following sentence from Torres’s original ruling raises my eyebrows:

… the investigation was initiated at the behest of the Court, itself, and cannot be described as the arbitrary action of a possibly overzealous prosecutor or runaway grand jury having ulterior motives.

Now, I’m not saying that Judge Torres has ulterior motives, but the mindset seems to be that courts are above such things. They aren’t; one can easily imagine situations in which the precedent that Torres is setting could lead not only to individual instances of judicial wrongdoing, but also to further usurpations of power. In researching this issue, one can nearly hear the pieces falling into place.
The court generated the protective order and was the most directly wronged party when it was broken. The court appointed the person to investigate the crime. And it is the court that has just deemed its own special prosecutor to have the power to coerce testimony. Unless I’m missing some important piece of the legal background — and discussion of such background is a notable absence in Torres’s reasoning — the step that this case has taken is the equation of a court-appointed special prosecutor with grand juries and investigators involved in cases in which the court is more clearly a disinterested third party.
ADDENDUM:
Incidentally, although this is a federal case, I noticed while researching that Rhode Island’s laws appear to offer a bit more license to judges when it comes to their orders and decrees. That might be something that we should look at as we push and pull the state toward governmental sanity.