As is so often the case, the inexplicable unjustices of history turn out to be entirely explicable once more of the details behind it are known:
On December 7, 1941, Airman First Class Shigenori Nishikaichi (c. 1919/20 – c. 10:00 am, December 13, 1941) (age 21/22), who had just taken part in the second wave of the Pearl Harbor attack, crash-landed his bullet-damaged plane, an A6M2 Zero “B11-120” from the carrier Hiryu, in a Niʻihau field near where Hawila Kaleohano (1912-1986), a native Hawaiian resident, was standing. Kaleohano was unaware of the attack at Pearl Harbor, but knew from newspapers that the relationship between the U.S. and Japan was poor due to Japanese expansionism and the U.S. oil embargo on Japan. Recognizing Nishikaichi and his plane as Japanese, Kaleohano thought it prudent to relieve the pilot of his pistol and papers before the dazed airman could react. He and the other Hawaiians who gathered about treated the pilot with courtesy and the traditional Hawaiian hospitality, even throwing a party for him later that Sunday afternoon. However, the Hawaiians could not understand Nishikaichi, who spoke only Japanese with a limited amount of English. They sent for Japanese-born Ishimatsu Shintani (an issei), who was married to a native Hawaiian, to translate.
Having been briefed on the situation beforehand and approaching the task with evident distaste, Shintani exchanged just a few words with the pilot. He paled; the pilot froze. Shintani left. The puzzled Hawaiians then sent for Yoshio Harada. Harada, born in Hawaiʻi of Japanese ancestry, and his wife Irene (both nisei), constituted the remainder of the Niʻihau population of Japanese ancestry.
Nishikaichi informed Harada of the attack on Pearl Harbor, a revelation Harada thought prudent not to share with the non-Japanese natives. Nishikaichi desperately wanted his papers returned, which he had been told should by no means fall into American hands, but Kaleohano refused to return them. For unknown reasons, the Haradas decided to assist Nishikaichi to retrieve his papers and escape.
In other words, those second-generation United States citizens proved to be considerably more loyal to their people than to their paperwork citizenship. They weren’t “every bit as American” as the descendants of the Mayflower and the Founding Fathers, they were Fake Americans and precursors to the post-1965 crowd.
Yes, nisei ultimately proved loyal and others didn’t. But the salient point is that no one could possibly know. Then, as now, the genuine loyalties of citizens of foreign descent and dual citizens simply cannot be assumed.
That doesn’t mean that the internment of all the Japanese citizens on the mainland was necessary; it was not because there was never any risk of an invasion of the West Coast. But the Niihau Incident does make it considerably easier to understand why internment was considered, and why few Americans had any problem with it at the time.