The interment of Japanese Americans on the Mainland in the US wasn’t some state secret and there was substantial support, little resistance and even cooperation by the Japanese Americans (don’t judge, see below).

The Japanese Americans weren’t immediately interred after Pearl Harbor. Consider that Congress unanimously (minus one absolute pacifist) declared war on Japan December 8, 1941 and Germany and Italy by December 14, 1941. Executive Order 9066 wasn’t issued until February 19, 1942.

There was a push in the West, especially in California (which at no point in history has ever been as tolerant as it believes) people and politicians to imprison and deport the Japanese, acting on long standing prejudices and economic interests (the Japanese owned land and were improving economically compared to poor whites, blacks and other minorities leading up to the war). However the Office of Naval Intelligence (ONI) and other assessments and government positions did not find a credible threat of the entire Japanese American population, and in fact the Japanese in Hawaii, a significant military asset, were never interred because there were so many that they were critical the economy and war effort.

The shelling of an oil field outside of Santa Barbara by a Japanese Submarine and the false air raid on Los Angeles in February was the final straw and with Pearl Harbor was used to justify interment. For Easterners it was no longer worth it to resist acquiescing these demands. By and large no one came to the aid of the Japanese, including other minority groups, who arguably either had no power either or wanted to advance at the expense of a rival (there is evidence of both).

Even the Japanese American Citizens League (JACL), the leading Japanese American community organization advocated and self-enforced complying with the order to show that they were good loyal citizens (which has been criticized by contemporaries including the modern JACL itself).

This isn’t to say they didn’t resist, it just looks different and incomprehensible to any lay post-1960s rabble rouser. This includes 442nd (see the late Senator Daniel Inyoue), Korematsu (who lost), changes even with the camps throughout the war. It was a valid approach and arguably it was widely successful given it was used in later years as an example of the Japanese as a model minority, and a significant reason for Hawaiian Statehood and eventual reparations during the 1980s.

By 1943 the War Department again assessed no threat, but was still under pressure by people and politicians to keep the Japanese interred. From a practical standpoint, reintroducing them during the height of the war would have created greater conflict on the home front (see Zoot Suits Riots in Los Angeles), probably violence against the Japanese themselves and in any event many didn’t have homes to go back to, in cities overcrowded to support the war effort. Instead freedoms and furloughs in town increased.

Edit: Almost every major newspaper (with only a few laudable exceptions) was also in support of Japanese internment and even advocated for the position before it was made. Notably this includes the Los Angeles Times which has since retracted it’s position,

https://www.latimes.com/la-ol-opinion-newsletter-japanese-internment-20170225-htmlstory.html