How did the US justify interning Japanese-Americans during World War 2?

Badly. Some of the military leaders in California were .... OK, racists, and decided that Japanese might be disloyal. After all, they might send signals to Japanese airplanes or saboteurs, right? So, many Japanese, and Americans of Japanese ancestry, were interned in rather unpleasant conditions away from the coast for the duration of the war.

It's astonishing, therefore, that when the Army asked for Japanese to volunteer for service, that so many did, and served so heroically. The 442nd Regiment, composed almost entirely of Japanese-Americans, fought in Europe and were the most highly decorated unit in American history.

As an interesting side-note, Japanese in Hawaii were not interned, even though Hawaii was far more densely populated with Japanese, and there were virtually no acts of sabotage or espionage among Japanese or Japanese-Americans.