The US government figured that if there were people that were spies on Hawaii then they would have left before the attacks, even so, there were still some instances of Japanese population segregation. And the reason that The Western Coast had the most of it was because the US expected the next attack on the US to be at San Fransisco or Los Angeles.
Step-by-step explanation:
Feel free to message me if you have other questions related to this, I'm a World War 2 nerd