Teacher cuts pupil's hair, mum files police report

It's called a deterrent. And this is a system where one is not required, because hair (short of being fluorescent) does not have any effect on anyone else.

Hair rules are in the same category as uniform rules. In Asia usually every school (both public or private) requires you to wear uniforms. It's the whole school spirit and togetherness thing. If you look bad you reflect badly on your school. Think of how bad it would look in an assembly (which we had every single day) where everyone's standing in neat rows and columns and someone has a different hair style.