A technical violation of the law was not prosecuted as a result of the exercise of prosecutorial discretion not to pursue a prosecution which was not in the public interest.
Grand juries are not a solution to this problem. One glance at the shockingly high percentage of cases in which grand juries indict should be sufficient to cast doubt that they are truly filtering out bad prosecutions. That the proceedings occur in the absence of the accused and therefore deprive them of procedural fairness is another problem.
The responsibility properly lies with the prosecutor.