Establishmentarianism is not promoting the idea of the magistrate being involved in church matters, but around them. That is, the church informs the magistrate of his duties with regard to the church, which is why he is given authority to call church councils, synods, etc. in determining the mind of God. He is not free to meddle in the church, but it is his duty to protect the church, suppress false religion (according to the rulings of aforementioned synods, councils, etc.), and enforce correction against public practice of false religion (again, as defined by the aforementioned councils, synods, etc.).