A. Because blaming the consolate deaths on Obama is a lot like blaming the Oklahoma City Bombing on Bill Clinton, something that nobody does. It was a terrorist attack, and blaming the President doesn't make sense.
B. I fail to see where it says he is standing by this claim. It was the initial claim, yes, based on information given at the time, but both the President, Mrs. Clinton, and the State Department are now backing the very claim that you just made, which is what makes sense.