Those that were around at the time have a different understanding than those that look at this affair from hindsight. It was not prejudice or bias, it was fear and hatred. The world was in a life or death struggle with the Axis.
The US was well aware of the Japanese behavior in Nanking shortly before. The US feared possible Japanese invasion of the west coast. After the Niihau affair on the very first day of the war, when the Japaanese-Americans turned on their adoptive country, FDR had every right to believe that should the Japanese invade the west, the result would be the same.
The Phillipines, East Indies, Singapore and others learned the hard way. Upon invasion the resident Japanese came out with armbands and supported their brothers. It was all arranged well in advance.
Accepting the only reason for internment as being "prejudice", plays well into the hands of those that wish to portray this country in the worst possible light. It is too simplistic, defies history and well suits the America haters.