Show me the law that says that.
Sadly --
Article II, Section 1, clause 5 of the U.S. Constitution states that:
No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident within the United States.
The fourteenth amendment to the United State Constitution provides an additional source of constitutional doctrine that emphasizes birth "in the United States" and subjection to U.S. jurisdiction at the time of birth, as the defining elements of citizenship (other than citizenship by naturalization):
"All persons born or naturalized in the United States and subject to the Jurisdiction thereof, are Citizens of the United States and of the State wherein they reside. . ." Amendment XIV, section 1.