So that begs the question, who is the Mass celebrated for? For the people, the pastor, the bishop, of for God? If liturgical law permits a priest to say Mass in a particular orientation, or a particular form, then that law exists for a reason.
If a priest is permitted under liturgical law to say Mass in a particular orientation then it really is nobody’s business, but God’s, what direction he faces. This is really not the business of the congregation, the bishop, the pastor as their wishes do not supersede liturgical law.
If people leave to go elsewhere because of this, then that’s there choice. The Church isn’t a democracy, and Mass isn’t said to please the people. Nor is the form of the Liturgy to be determined by the personal preferences of the diocesan bishop. It really is entirely up to the individual priest celebrating the Mass (within the terms of what is permitted by liturgical law).
The priest does act in a vacuum. He acts to draw souls closer to God. And so, his primary responsibility being to those souls, his personal desire to celebrate one way or the other is tempered by that duty.
The bishop is the chief liturgist in his diocese. As such, if there are variations, it is within the bishop’s general authority within those variations to determine which of them shall be followed; the priest in not so authorized in general. To reduce the question to one of whether or not ad orientem can be practiced by any priest simply because it is in the GIRM is to miss a few other issues, the bishop’s authority being one of them.
But more specifically, to answer your dicta that it is nobody’s business how he celebrates Mass, as long as it is within something within the GIRM is to ignore that he has a higher duty to his parishioners than to act as if the Mass were his own personal statement of rubrics. He is not there to say Mass primarily for his own personal edification.
And that is not to weigh in on a different question, as to whether ad orientem is the preferred orientation. If ad oreintem is within the rules of the GIRM, then it is up to the bishop to lead on that issue; and if the bishop does not so lead, then one can take it up with Rome - perhaps not the most productive use of one’s time, as Rome has not seen fit to determine that it is going to enforce such a direction.
There are always going to be some parishes which would prefer ad orientem by a significant majority. That is well and good, and doing so (assuming one is not running afoul of the bishop) works for the increase of their spirituality. The reality is that the vast majority of Masses said appear to be ad populum in the US. While Rome may appear to favor ad orientem, they also appear to favor Latin, Greek and Hebrew in the prayers of the Mass, as well as Gregorian chant; and they should be presumed to be knowledgeable as to the actual practices. Which is to say, the actual practices are within the legitimate gamut of the law, and appear to be within the liturgical direction that the bishops, in their dicoeses, determine. We may think and feel mightily, that otherwise is better. But the “we” who do so are in a distinct minority, as would be the priest. His primary mission is to get souls to Christ, not to determine what is liturgically better, or what suits his fancy better. When he is ordained bishop, he will have time enough, and authority enough. And at that point, even more responsible for leading souls to Christ.