One of the reasons that schools favor zero-tolerance policies is that they remove the school from reasoning, critical thinking and individual evaluation. They can just look up the punishment and administer it. Neat and tidy.
If a parent questions the punishment, the school can say it's out of its hands.
We expect our schools to teach children critical thinking and to interpret what they read using their own individual views. Why should we expect less from school administrations? Why are they not held to the same standards of reasoning and critical thinking?