A group of Japanese-Americans who supported the government's side (and tried to suppress anyone who protested) did arise -- the JACL. As for why people would do this ... there are a bunch of possibilities. Fear that if they were seen to be rebellious, the government might crack down further? Vestiges of militarism in those who'd grown up under the Japanese military dictatorship? The desire to erase the accusations of "traitors in our midst" by being superpatriots? Loyalty towards powerful personalities who were involved? Desire for power over others? (some of them worked with the government on isolating individuals felt to be negative influences)
There was considerable division within the community about how to respond to the internment, and when people were asked to take a loyalty oath, some of the pro-constitution activists were indignant that such a thing would be proposed (would the patriotism of any other Americans be challenged in such a fundamental way?). Others felt they should just sign the oath and get it over with, and not make such a big deal in case they were made out to be troublemakers by the non-Japanese who wanted to expel them from the country entirely. As far as I'm aware, some of these divisions (even within families) continued right up until the present day ... many of the people who were interned are elderly or have died, but their kids can often tell you about some of the stuff that went on.
"In San Francisco, magazine publisher James Omura protested the imminent expulsion of all Japanese Americans from the West Coast. Three individuals in California and Oregon defied military curfews imposed only on persons of Japanese ancestry. But their voices were overwhelmed and opposed by the widely publicized statements of the Japanese American Citizens League and its national spokesman, Mike Masaoka. The JACL pledged the cooperation of its members, and the government saw in JACL a group with which to work to carry out the President's program of mass exclusion and detention. The JACL's motto, "For Better Americans in a Greater America," proclaimed its desire to prove that its members were loyal to the U.S. and not Japan. To that end, JACL provided names of Issei leaders to the FBI, opposed all test cases, and urged the creation of suicide battalions to create unquestionable proof of loyalty in blood."
http://www.pbs.org/itvs/conscience/http://www.resisters.com/