Barack's bow was not the biggest thing in the world. It also was not so much about U.S. - Saudi relations as it was about all observers in the Middle East and elsewhere who watched and formed this impression: weak. We promote democracy and we bow to a King?
I suspect Barack just did it on the spur of the moment, even maybe catching himself mid-bow "Uh oh, I think I better straighten up".
Conservatives look at Barack and see someone who doesn't fully get what makes America great. Barack would not faux pas over the symbolism of a black man bowing to a white man; would understand the symbolism of a woman bowing to a man; yet lightly forgets the blood which has been shed - is being shed at this very moment - so an American need not bow to anyone's King? It's not that Barack doesn't understand the symbolism. It is, rather, the symbolism appears further down his priority list (so far down that he momentarily forgot about it) than it would have been for almost any other POTUS in history. Even writing this, I notice myself becoming angry about the apparent casualness regarding American sacrifice and principle.
It's not that the bow will have any immediate practical effect. It is, rather, sort of like a filthy bathroom in a restaurant: what ELSE about America's greatness is Barack casual about? If Barack doesn't understand - deep in his bones - what makes America great, what principles is Barack basing his decisions upon?