they were unecessary. they could have let them stay in their houses and just have the police check up on them daily and monitor their calls.
you could argue that they didn't speak english so no one knew what they were saying, the japanese really dominated East Asia (incl. China) for like a hundred years so people were afraid of them.
Japan is the tough guy of east Asia remember their Samuri Mindset? The Chinese call them the dwarf pirates.