My main question is, can Nationality on a US passport state anything other than "UNITED STATES OF AMERICA"
No, since even before the 1856 enacted passport statute, passports issued by the Secretary of State for travel outside the United States, generaly contained the text that the person is a "Citizen of the United States".
The United States Passport: Past, Present, Future - United States. Passport Office (1976) - Google Books
In 1856 Congress enacted what remains today as the basic passport statute.
This law provided that the Secretary of State be authorized to grant and issue passports, and cause them to be granted and verified in foreign countries by diplomatic and consular officers of the United States under such rules as the President might prescibe. No one else was to issue passports, and they must be issued to none but citizens of the United States.
The Act of 1856 also made it a penal offense for a consular officer to issue a passport to anyone who was not a U.S. citizen.
On a side note, if it's always the same, why is it mentioned?
I would appreciate any sources.
This is due to the norm set out in the 1920 League of Nations Passport Conference ¹, where it contains the wording:
in the event of a passport being issued by a Government to persons other than its nationals.
Until 1914 passports could be issued to non-nationals and sometimes done for residents.
During the 1920's, this praxis was replaced with the general rule that passports should only be issued to nationals (introduction of the 'foreigners passport' in 1922).
Some countries still have different grades of citizenship.
A UK passport can contain British citizen or subject, with the holder being treated differently based on that entry.
¹ the present day ICAO recommendations are based on these norms developed during the 1920's