Trivia: When Killian is talking about the next possible contestant, he points out he's 120 pounds and wouldn't last 30 seconds. This is somewhat making fun of how they used Arnold Schwarzenegger to portray Ben Richards. In the book "The Running Man" (which has a very different structure), Ben Richards is described as "scrawny", the complete opposite of Arnold.
Trivia: Ben says to Killian "I'll be back", his famous line from The Terminator.
Answer: They needed a scapegoat, someone to blame. The government controlled the people through violence, but at the same time tried to convince the people the government was fair (i.e. Runners who made it through were given their freedom when in fact they never left the arena alive). The people at Bakersfield were still massacred (that part wasn't made up) by the government in order to control the riot, but just not by Ben. But people would just riot more if they knew the government ordered their killing, so the footage was faked to make people think Ben acted alone and against orders. And to continue the ruse, they convicted him to appease the people and prevent them from rioting.
Bishop73