That is because he is a Muslim. No "Christian" would bow to the King of Saudi Arabia, who is the keeper of the Islamic faith. But any Muslim absolutely would.
And think about how many years ago he wrote that in his Ayers ghostwritten book,how "prophetic".
Here's an excerpt from The Audacity of Hope concerning that passage. Given Obama's legendary wordiness with his 17 minute answer, it's not hard to believe he wrote this part. The portion below is only two sentences. He's the king of the comma.
In the wake of 9/11, my meetings with Arab and Pakistani Americans, for example, have a more urgent quality, for the stories of detentions and FBI questioning and hard stares from neighbors have shaken their sense of security and belonging. They have been reminded that the history of immigration in this country has a dark underbelly; they need specific assurances that their citizenship really means something, that America has learned the right lessons from the Japanese internments during World War II, and that I will stand with them should the political winds shift in an ugly direction.