Short answer. The US Constitution enshrines the Grand Jury process, which was always a secret process.
Long answer. Grand Juries were originally designed to protect citizens from unjust accusations. The British recognized that the mere suspicion of criminal accusations could damage a person's reputation. So, to protect a person from having their reputation smeared, they invented this process that would protect a person until they were actually accused. It was a "shield"
At some point in time, prosecutors began to figure out they could use the Grand Jury as an investigative tool. They could protect witness identities, and force their testimony, thereby preventing the "bad guys" from finding out who was testifying, and intimidating them. It became a "sword".
No one has ever done an empirical study of the accuracy of claims of witness intimidation or need. The Supreme Court has established a series of rules regarding the Grand Jury. It is not just defendants who cannot see Grand Jury materials, no one can without a court order. Federal rules do not allow attorneys, or case agents, in the Grand Jury.
Prosecutors are not obligated to offer the Grand Jury evidence consistent with innocence because the only question is there a probability of guilt; after all people are presumed innocent, even when they are charged by a Grand Jury.
Grand Jury testimony is not admissible in a trial. No one can be convicted on statements that were made in front of the Grand Jury. The question of whether or not a prosecutor can be held responsible for knowingly offering false testimony to the Grand Jury is still open. If I were betting, I would bet against it.
The grand jury process is held in secret for two main reasons. First, an investigation may be going on for some time without the target of the investigation knowing that he is under investigation. Second, the secrecy is intended to avoid damaging the reputation of someone who is investigated but no charges are ever brought. The prosecutor is not required to present both sides. If there is probable cause to believe that the defendant is guilty, that is a sufficient basis to get an indictment. I think most courts would take the position that any abuses in the grand jury are cured by a fair trial on the charges. If he knowingly presents false testimony, that could be viewed as a fraud on the court and could be a basis for dismissal. It would be almost impossible to prove, however.
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Some of what you write about grand juries is correct, but some is not correct. When I was a prosecutor in the federal system, I often heard the remark, "You can indict a ham sandwich," so certainly you are right a prosecutor usually can indict anyone for anything, assuming that his superiors don't review his work very carefully. It is also true that grand jury proceedings are protected under Rule 6(e) of the Federal Rules of Criminal Procedure. This secrecy both protects grand jury witnesses from intimidation and targets who have not yet been accused on any criminal offense.
However, it is not true that grand jury proceedings remain secret under all circumstances. For example, if a witness who testified in front of the grand jury testifies at trial, the government must disclose any transcript of that witness's grand jury testimony under what is known as the Jencks Act. Also, if the government knowingly fails to present exculpatory information, the defendant may file a motion to dismiss, although under Supreme Court precedent it is almost impossible to prevail in such a motion.
On the other hand, at least when I was with the Department of Justice, targets were always permitted to testify before the grand jury if they so wished, and better prosecutors would seriously consider presented exculpatory evidence, since if a prosecutor cannot convince a grand jury to find probable cause despite such evidence, he should not be seeking an indictment that will require proof beyond a reasonable doubt,
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