A “National Historic Site” is not a “National Park”.
Hindsight is always 20/20
People tend to forget, while we look back at it as inevitable the American government would prevail in its prosecution of the war, in 1942 this was by no means certain. It was an all out effort, a 100% commitment by everyone. They rationed everything.
You couldn’t even buy a tube of toothpaste without turning the empty one back in to your friendly grocer, who counted it against your points. There is no way the i would have wasted any resources interning thousands of people unless they truly thought it was necessary.
There’s no other explanation that fits the facts. They weren’t going to take any chances, because they wanted to succeed in their objective. It illustrates the potential danger they feared, of having significant numbers of people within a political unit who may decide to cause trouble in one form or another. Enormous resources and planning were involved, men and material that could have been allocated to other direct methods of prosecuting the war.
I’m not excusing what they did, but I understand why they thought it was necessary at the time. There was no way to know for sure at the time.