And if Obama is covered by the law that you posted, it does not define "natural born citizen" as defined in the U.S. Constitution, which determines eligibility to be a United States president. The law you cited only determines U.S. citizenship.
That provision deals with employment related to the US Government or International Organization listed under noted law.
That's incorrect. Read, for example, the following post:
http://www.freerepublic.com/focus/news/2036156/posts?page=94#94