This wasn't the prosecutors decision on this one, this was the DOJ.  They didn't prosecute because there wasn't a case.  Freddy Gray died at his own hand, he thought if he was injured he could sue.  Problem was he did too good a job on himself.  Seat belts haven't been utilized in paddy wagons for eons.  Scratch it up as another loss and concocted story for the Black Lives Matters folks, maybe they can embellish the Michael Bennett story so more and live another day.