That's the whole point of the grand jury you're alluding to. They will decide if the facts of the case are worth presenting to a trial jury. This process happens for any criminal case including civilian shootings. I'm not really sure what you think is different here than every other court case tried in the United States. Civilians are let off all the time for shooting other civilians, i.e. home invasion cases, auto theft involving armed robbery where somebody defends their person and property, etc. If a grand jury finds the evidence is insufficient for a case, why should a public servant be held to a different standard than a civilian? Are the rights guaranteed by the Constitution suspended for police officers?