The back of trucks are not for carrying nor designed for passengers but only items or materials. There are no safety devices in the bed of a truck to restrain a person(s) if an accident were to happen. Common  sense would dictate this fact. 
As for the individual getting his head stomped in after a stop for excessive speed. I highly doubt that is what occurred but if force was used in the situation what did the person do that warranted any sort of response from the Troopers that required the use of force.