I used to think the same, but it is not true. It must be made in the US and be made from at least 51% corn.
The Federal Standards of Identity for Distilled Spirits, codified under 27 CFR ยง5.22(b)(1)(i), states bourbon made for U.S. consumption[22] must be:
Produced in the U.S. and its Territories (Puerto Rico), as well as the District of Columbia[24]
Made from a grain mixture that is at least 51% corn[25]
Aged in new, charred oak containers[25]
Distilled to no more than 160 (U.S.) proof (80% alcohol by volume)[25]