Maybe they think it would be easier to deal with the protesters after a No Bill than to let this thing drag out even longer with a trial that would end in an acquittal.
From the few reports coming out of the Grand Jury, the prosecutor has been presenting witnesses that would exonerate Wilson; not usually the approach a prosecutor takes.
It seems like an indictment gives the prosecutor some leverage in getting a plea bargain, but I doubt Wilson would make any deal. He has to know, as does his lawyer, that a conviction with what little evidence they have is almost impossible.
So, I think they're mostly trying to avoid a trial and acquittal. No easy way out for the county prosecutor, but I think they're looking for the quickest way out.
You don’t think a jury from St. Louis country wouldn’t turn in a false “guilty” verdict just to avert a riot or seek some sort of vengeance? It would be very dangerous for the prosecutor to bring trumped up charges on the belief or hope he would be acquitted due to the lack of evidence.