My reading is the most likely "it" in context is restricting the freedom of movement of ethnic minorities that he mentions immediately before: "They went throu

My reading is the most likely "it" in context is restricting the freedom of movement of ethnic minorities that he mentions immediately before: "They went through a whole list of things -- they couldn't go five miles from their homes, they weren't allowed to use radios, flashlights," Trump said. Maybe he means "Hey, if that's legal than my policy is less bad and therefore legal too" which is only marginally reassuring. Citing internment as a necessary tactic in wartime as a moral justification for your policy is plenty worrying on its own. "Look how generous I am- I'm not actually interning people, just X. I don't like it, but it's a time of war, so it's necessary."