Ultimately, the point is this: if there was a grand jury proceeding concerning Michael Brown for some crime (whatever crime you might think of for which there could be an absolute defense like self-defense), the prosecutor would not have endeavored in any way to have put evidence concerning potential defenses available to Mr. Brown into the grand jury proceedings; the grand jury would have only heard the evidence tending to incriminate Mr. Brown.
That is what the grand jury system is intended to do.
The grand jury is really intended to test the strength of the state's proof on its best day and determine if that proof would be sufficient (without regard to any controverting evidence) to sustain a conviction. This is the real import of Justice Scalia's opinion in Williams, which expressly states that the prosecutor is not required to bring forth exculpatory proof in grand jury proceedings even where that proof is directly contrary to the State's theory. That is true, cons utionally, because the grand jury's role is not to determine guilt or innocence, but to simply decide if there is enough evidence of a crime to make conviction possible.
It's not intended to be a place where the merits of the case as a whole are worked out and resolved, primarily because grand jury proceedings aren't burdened (in the main) by the rules of evidence or procedure and without judges there to control the proceedings. That's why we have a second step in the prosecution of any individual -- a trial in which the rights of the accused are substantially greater.
I have said before (and reiterate) that I don't really care about the outcome of the Wilson grand jury; I think the result was probably right. But I do think that there are legitimate concerns when the manner in which a case against a law enforcement officer is presented to the grand jury is considerably different (in ways that are more helpful to the defendant) than the manner in which a case against another citizen would be presented to the grand jury.
What makes the Wilson case unusual and worrisome to an extent is that in that instance, the prosecutor actually tried the case to the grand jury and went to some length to provide the grand jury with evidence concerning Officer Wilson's defense. Those efforts are unusual and suggest (to some, at least) that the prosecutor -- who has a clear conflict of interest when faced with the prospect of seeking indictments against those he and his office work closely with -- treated the grand jury proceeding as a de facto trial. If those efforts would not be used in other cases, there's very real danger of abuse (that is a more general matter of prosecutorial discretion and not a specific criticism of the Brown/Wilson proceedings). A prosecutor might bring a thin case burdened by substantial exculpatory evidence before the grand jury against a citizen, wholly ignore the exculpatory proof favoring the citizen in that proceeding (leaving the grand jury entirely in the dark), obtain an indictment (particularly since indictments are quite easy to obtain), and use the indictment as leverage to get some sort of a sentence or punishment against that person. The same prosecutor might choose in another case to present his thin case but offer the grand jury the exculpatory evidence or the proof bearing on a defense as well to suggest a no-bill. When the iden y of the accused in the first scenario is a poor, uneducated black man and the iden y of the accused in the second case is a white cop or the white son of a fellow country club member, the concerns for the fairness of grand jury proceedings in general become pretty significant.