One could take the long term view. If Obama speaks only with clergy who support his positions, how will he ever hear the other side? If Warren and other pro-life clergy refuse to speak to him because of his views, how do they have any hope of persuading him to see the issue from another perspective?
As I recall, it was a minister who regularly picketed at an abortion clinic who befriended and eventually converted Norma McCorvey (the original Roe) to a pro-life position. Had he refused to speak with her because of her historical role, she might still be a lost soul on the issue of life.
It is the mission of the men of God, the men of Christ, to preach to all God’s children, not just those who agree with them.
Speaking WITH is one thing. Giving an invocation where the entire reason you were selected was the “evil” man’s political benefit is another.