Been away a while and probably not the best first post on return, but...
The whole grand jury affair does not sit well with me in the first place.
Historically speaking, grand juries were there to judge the ability of a prosecutor to build a case. Before there were professional, appointed public prosecutors many cases were brought by amateurs (often police officers or the victims/interested parties themselves) or hired lawyers whose abilities may not be tested in that area. Any such party could bring a bill of indictment before the grand jury, who then decided if there was a case to be answered. That is, the prosecuting party had to prove that what they claimed to have happened was a crime, and that they had some evidence that it happened. There was no defence offered. The jury had the power to investigate, but generally the test was that the evidence didn't immediately collapse as absolute fiction and that the act being described was actually a crime. At that point the prosecuting party was granted the power to represent the state in prosecuting the accused in court.
Which is to say they were basically a gateway to filter out incompetents from bringing nonsense cases.
The requirement for a grand jury to clear a case before prosecution is written into the Fifth Amendment and several states have legislation requiring them in some cases, but outside that they are largely redundant given professional prosecutors and preliminary hearings in front of judges. It's rare you have to test a state prosecutor's competence to prosecute a case, and so a brief hearing of the charges and evidence to make sure they aren't a complete farce is all that is generally needed.
Modern grand juries (outside federal cases) tend to be rubber stamps or political tools for prosecutors. The indictment rate is near perfect; usually around 98-99% for the state statistics I've seen, with a federal rate of only
11 cases turned down out of 162,000 in 2010.
It's not hard to see why when you consider that such hearings are basically a trial without any defence, and without even requiring a guilty verdict; simply a majority who believe there is a case that needs answering. A prosecutor who wants to bring charges and can't put such a case together would simply never get into that position these days. At least some minimal legal competence is required to rise to that level, and that's all that such a task asks. If that, given it's the prosecutor who advises the jury as to the legal requirements and standards of indicting the accused.
But then the grand jury can also be used as a political shield. This makes sense when there is a person with considerable influence being investigated. Running the investigation and prosecution in the name of an anonymous and protected jury rather than a politically appointed prosecutor or state employed police department makes sense in such cases, and offers the prosecutor a degree of independence they may not otherwise feel.
It can also offer distance from lose-lose cases. A case where the prosecutor would be lambasted for either action or inaction can be dodged by passing it off to a grand jury, while the prosecutor still retains the ability to get the outcome they desire from the jury by exercising the same near certainty of indictment if they want it.
Quite frankly, that's what I consider to have happened here. The case presented was not the sort of case you expect a prosecutor to bring in front a grand jury, and almost seems designed to have failed.
One strong indicator (IMO) was Willson's testifying. Not the content of his testimony, but the fact of it. While a Grand Jury can compel witnesses to testify it does not overrule the Fifth Amendment; you can never force them to testify if they may incriminate themselves. Because of this you hardly ever see defendants brought in front of a grand jury. The fact that he would be called at all is weird, that he offered testimony that sounded like defence testimony
begging to be cross-examined by a prosecutor - despite being presented to the jury by a prosecutor - is just absurd.
There is also the simple fact that there were never specific charges presented for the jury to indict on. Usually a prosecution case is by the numbers; here are the points that must be supported by evidence for there to be a crime. To not have that template and the evidence laid out in such a way it would be considerably harder for a jury to find a count that matches the evidence.
This isn't exactly uncommon though. That 538 link above pointed out that despite rarely failing to indict, the majority of grand juries brought to hear charges against police officers return no-bill. Given that cases involving officers are the perfect example of lose-lose cases for the prosecutors (who have extreme pressure not to convict the officers they work with) these are the exact cases I'd expect to see thrown away in farces such as this one.