Because the internment broke several conditions of the Bill of Rights of the Constitution (in the United States). The same thing happened in Canada. I've lived in 2 communities that interred the Japanese during WWII. I find they have varied in their response to what happened to them during the war, but for the most part, they are model citizens. They have been very forgiving of their treatment.
It is important for every citizen to remember to defend the Constitution. The rights of citizens cannot be taken away on a whim. Every safeguard must be fully exercised or the Constitution is just a piece of paper.
New Citizens should remember that rights are conditional. If treason is involved the Constitution will not provide adequate protection. If obedience is required, it should be given. That's the balance between the Bill of Rights and allegiance to another country. The internment should never have happened and wouldn't have in normal times.