Fr. Gerald Murray: "I'll say this again... No bishop can prohibit Ad Orientem celebration of the Mass in his diocese beyond his powers, because it is foreseen in the Roman Missal, and the Dicastery of Divine Worship has affirmed in private correspondence in the past that this remains a legitimate way to celebrate Mass. Sad to say, many bishops ignore it, and they won't like it to hear it said, but there and by, why in the world would you want to prohibit what was done for 1300 years or more?"
When the Novus Ordo came out, it stated (for the Orate Frates) the priest TURNS to the congregation, meaning that it was taken for granted the priest was celebrating the Mass AD ORIENTEM.
It was taught in theology schools since Vatican II that the bishop is the "chief liturgist" in his diocese. Usually a task he performed with the consultation of some professional liturgist [mostly female]. Hence the reason why the N.O. Mass is different from diocese to diocese.