There seems to be some confusion in your statement. Harley-Davidson motorcycles are not made by a Japanese motor company called Nihon. Harley-Davidson is an American motorcycle manufacturer that was founded in 1903 and is based in the United States.
Harley-Davidson motorcycles are traditionally associated with American manufacturing because the company has a long history of producing motorcycles in the United States. While they do have some international production facilities to serve global markets, the majority of Harley-Davidson motorcycles are still manufactured in the United States.
It's worth noting that Harley-Davidson has had collaborative ventures with international companies in the past, but these collaborations typically involve partnerships for specific purposes such as distribution or component production. Nonetheless, the core design and manufacturing of Harley-Davidson motorcycles remain under the control of the American company.
In summary, Harley-Davidson motorcycles are called American-made because the company is based in the United States, and the majority of their motorcycles are produced in the U.S.