It’s the political climate. Same reason they havent indicted other perps.
They dont want to wrongly file on the cops as it would lead to trials and each aquittal would result in the same pillaging all over again.
So, your attempt at reasoning is: The GJ decided not to charge the cops (resulting in riots) because if they had charged them and the cops were acquitted by a jury trial, it would result in - - - riots. Gotcha!
