Matt 18:15 If your brother sins, go and show him his fault when the two of you are alone. If he listens to you, you have regained your brother.
18:16 But if he does not listen, take one or two others with you, so that at the testimony of two or three witnesses every matter may be established.
18:17 If he refuses to listen to them, tell it to the church. If he refuses to listen to the church, treat him like a Gentile or a tax collector.
Knowing about that particular bishop, I'm sure they did not talk to him first, but there is no way that he could not have known the position of the Church on abortion and the fact that he was PUBLICLY opposing it.
The public nature of his action is what makes all of this different, and in fact what makes the situation of politicians different. They are publicly advocating, from a position of leadership, for something that is opposed to the teaching of the Church. And they know it perfectly well.
Maybe if more of them were called on it publicly, they might think twice.
How exactly did the early Christians treat Gentiles and tax collectors? And by Gentiles, did they mean non-believers (non-Christians)? Gentiles also means Christians.