Based on all of the evidence presented to the jurors, they made the right decision. Let's summarize:
Brown just stole from a convenience store
Then threatened the clerk
When walking in the street, the cop told him to move
His reply was f'off
Cop said stop and started to get out of the police car
Brown pushed the door close and punched him in the face
Then tried to grab his gun
First ran away, then turned back and charged
Then he was shot
Pretty simple decision here.