Here are some collections of firsthand accounts to answer the "what were they like" question. http://www.tellingstories.org/internment/ http://www.densho.org/

As to why, the short version is that the American government believed Japan's own PR hype. You see, Japanese propaganda at the time emphasized the idea that all Japanese were shimin (subjects) -- sometimes the term koumin ("People of the Emperor") was used as well. This ideology emphasized the idea of loyalty to the Emperor and the divine nature and mission of the Japanese race. That's the tl;dr version, anyway -- there are better summaries out there if you want to delve into the intricacies of Japanese propaganda.

Now, it's debatable how much this applied to Japanese nationals with citizenship, but certainly in the case of naturalized Japanese-Americans it was not applicable. However, racial discrimination combined with a fear of the Japanese -- who had gone overnight from a minor power to a genuine threat in American eyes after Pearl Harbor -- caused many Americans to start believing that all Japanese were caught up in this shimin mindset, including the ones who had only ever lived in the US. I'm also given to understand that Japanese communities in the 1940s tended to be less assimilated than Italian and German ones, though US history is not my specialty so I could be wrong.

Perhaps the best way to understand it is to think about the nature of American propaganda during the war. Our European propaganda emphasized the idea that we were fighting against a political movement -- we were fighting a war against German and Italian ultranationalism. However, propaganda in the Asian theater emphasized the idea of a war against the Japanese race. In this line of thinking, you don't become any less a potential enemy by having American instead of Japanese citizenship, since the "enemy" is not ideological but cultural or racial. There's a good book on this subject of race in the Pacific War; John Dower's War Without Mercy.

Tl;dr: Internment was driven by the perception that loyalty to the Emperor was so ingrained in Japanese racial and cultural identity that any Japanese-American could be a potential fifth columnist.