Ignoring the emotionally charged argument above, I'm just gonna throw in the point that there was no cross examination at the cop's trial. Literally none. How can it therefore be considered anything but a show trial?
We also see from the hospital reports that Darren Brown sustained no physical injuries and yet Brown stated he was punched in the face twice. He also compared Mike Brown to Hulk Hogan. Major discrepancy here.
The cross examination being of the defense side, which would be Wilson's side. A grand jury is a prosecutor's playground. The prosecutor probably looked over the evidence and saw little chance of an indictment coming out of it, but put it to the grand jury for the purpose of CYA. Lots of arrests never lead to the "let's get an indictment" stage because many prosecutors utilize discretion.