Ben Richards: I'm not into politics, I'm into survival.
Ben Richards: Damn that Killian!
Amber Mendez: What's wrong? You should be happy! We're officially dead, we can go anywhere, do anything.
Ben Richards: No! Don't you understand yet? They'll never let us out of here alive, they can't afford it! They'll get the police, the army, and hunt us down like dogs! Off camera, of course.
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