Quote me the law on that one, if you can...
I think it’s the Naturalization Act of 1790.
“The Act also establishes the United States citizenship of children of citizens, born abroad, without the need for naturalization, “the children of citizens of the United States that may be born beyond Sea, or out of the limits of the United States, shall be considered as natural born Citizens”.”
http://en.wikipedia.org/wiki/Naturalization_Act_of_1790