Morning Alexkoepp and welcome to the forum.
The US does, indeed, collect a bonus of 30 whilst at war with Japan. You will discover it needs this income or will not stand a chance fighting Japan. India and Anzac will be struggling on 3-10 income a turn, whilst Japan is collecting 60-70.
America is all that is stopping Japan from winning. The income boost represents the East Coast, which is not represented in Pacific.
I hope I have helped.