"Although there is evidence of potential violations of the statutes regarding the handling of classified information, our judgment is that no reasonable prosecutor would bring such a case"
Statement by FBI Director James B. Comey on the Investigation of Secretary Hillary Clinton's Use of a Personal E-Mail System
When the FBI Director, himself a former prosecutor, used those words, he was giving expression to the principle of prosecutorial discretion. This principle requires a prosecutor to exercise discretion whether to seek an indictment and bring charges against an individual. This discretion or judgment is an important part of our legal system. Once an indictment is filed, the wheels of justices start to roll and, whatever the result of the charges, the life of the individual charged is changed. An indictment, even if there is ultimately no conviction, may mean loss of employment, public and private scorn, not to mention the cost of hiring lawyers. There is the fear of conviction and loss of the freedom we all treasure but sometimes take for granted.
The Courts have long recognized that the decision whether or not prosecute is best left to the prosecutor. As the United States Supreme Court instructs:
"[S]o long as the prosecutor has probable cause to believe that the accused committed an offense defined by statute, the decision whether or not to prosecute, and what charge to file or bring before a grand jury, generally rests entirely in his discretion." Bordenkircher v. Hayes, 434 U.S. 357, 364, 98 S.Ct. 663, 668, 54 L.Ed.2d 604 (1978). This broad discretion rests largely on the recognition that the decision to prosecute is particularly ill-suited to judicial review. Such factors as the strength of the case, the prosecution's general deterrence value, the Government's enforcement priorities, and the case's relationship to the Government's overall enforcement plan are not readily susceptible to the kind of analysis the courts are competent to undertake." Wayte v. United States, 470 U.S. 598, 607, 105 S. Ct. 1524, 1530, 84 L. Ed. 2d 547 (1985)
When FBI Director Comey said "... our judgment is that no reasonable prosecutor would bring such a case...." he was referring to the factors described by the Supreme Court, and in particular "the strength of the case..."
This exercise of discretion comports with a recent statement of the Supreme Court in a case it characterized as "distasteful" and a "tawdry tale." The Court, taking the unusual step of disagreeing with the prosecutor's exercise of discretion, reversed a conviction because:
"... our concern... is ... with the broader legal implications of the government's boundless interpretation of the federal... statute.
Chief Judge John G. Roberts in McDonnell v. United States http://www.supremecourt.gov/opinions/15pdf/15-474_ljgm.pdf