The Citizenship System.

The promotion of competition amongst the young Supreme Beasts is why the Supreme Alliance rewards those who eliminate other Supreme Beasts with half of the points of their targets during the rite of passage. But none of them can be killed. So in this way, a safe environment that encourages competition has been created for the efficient distribution of resources. 

That's what the rite of passage is for the Supreme Beasts. It is another thing entirely for the inhabitants of the plane who have to be indoctrinated by peaceful coercion or forceful enlightenment. 

"The Supreme Alliance has thought this through. We are not only fighting the inhabitants of the plane but each other too. No wonder some people will survive in the plane. We won't be able to focus on them if we have to worry about each other. They want to temper the young while also leaving a way out for the inhabitants of the plane."