What reason did the U.S. government give interning U.S. citizens?
Do you mean in the 1940s, or since 2001?
In either case, though, the answer is the same - it is for the safety of the country. Whether this answer is actually true, well, I don't think there was a single act of sabotage committed by an American citizen of Japanese descent in the 1940s. (One of the early court cases on loss of US citizenship involved a dual US-Japan citizen who served in the Japanese military, and mistreated American prisoners.)
In the post-2001 case, well, let's see - there have been several hundred of thousands of murders of American citizens during that time, nearly all of them by other Americans. Does this constitute some grave existential emergency, requiring martial law, suspension of constitutional rights, etc.? Apparently not. Despite the continual slaughter of Americans by other Americans, the police cannot just lock up murder suspects forever without charge, they cannot assassinate suspects, torture them to get information, etc. They can't even listen in on the telephone calls of murder suspects without permission from a judge.
But, if less than 1% of those murders are committed by Arabs, that's an entirely different story. For that, you can start wars, kill hundreds of thousands of people, throw people in prison without charge for ever, assassinate suspects, torture prisoners, and so on.
So if there are 100 people out there who might murder you, the question you need to ask is - is one of them Arab? If so, then you are in grave mortal peril. Eliminate the Arab, so that only 99 Americans who might murder you remain, and then you are safe.
All murders are equal, but some are more equal than others.