Because they were lazy?
Because they assumed, that because it is 2016, Google exists (relieving Will88 and who knows who else of a trip to the Public Library)?
From the Act:
And 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
An interesting fact is that the 1790 Naturalization Act is the only piece of US legislation (other than the Constitution) to use the term "natural born citizen".
“And 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”
The quotation demonstrates the children are naturalized citizens who are not natural born citizens.
-PJ