Because separation of church and state, that's why. Because it would violate the Establishment Clause if religious employers could control what healthcare services their employees could access REGARDLESS of the employees' religious status. And since the insurance is state-mandated, the state damn well has an interest that can't be overruled by church doctrine. In this specific case even moreso, since the Catholic prohibition on birth control applies ONLY to Catholics, even in the wording of their own doctrine.