Back when many if not most prosecutions were private, the primary function of a grand jury was to squelch them if they were frivolous or unfounded. Now they don't happen, the grand jury is a mostly useless vestige. In Federal cases, they are constitutionally required, so prosecutors abuse them. However, while the practice of placing only evidence hostile to the defendant before a grand jury is permitted, it is not a mandate; in fact, the US Marshals' manual tells prosecutors to present the grand jury with exonerating evidence. A prosecutor who uses a grand jury to quash a prosecution the mob is demanding is acting far closer to the original intent than one who uses it as a rubber stamp.
Just because somebody practices criminal law, doesn't mean he knows this. I work with dozens of analytical chemists who don't know the history or first principles of the techniques they use.