The key issue in this case was discrimination. The school allows all other sorts of clubs to meet on school property, but denied the right of the Christian club to meet. No child could attend the Christian club without parental permission, so it wasn't as if children were becoming the victims of a state-imposed religion.
The usual suspects were appalled - from the New York Times editorial page to the ACLU and Americans United for the Separation of Church and State. It appears a greater threat to them that God might be spoken well of on public property than it is God might be spoken ill of during regular school hours as all sorts of godless ideas are presented and taught as fact.
While this is an important victory for freedom, it would be better if Christian children were taught at home or in Christian schools.