Wow, I didn't know there are people like my Mom in Japan. I was born to a Japanese father and a third generation Japanese American mother, and was raised in Japan till I was 15. Although I do have some pleasant memories in Japan, most of my growing pains were due to the sense of marginalization I felt for being different from my peers. In a country that places so much conformity, I felt culturally marooned. Frankly, it's very hard for me to understand why a Japansese American would want to live in Japan, but this film sounds definitely interesting.
Peter Mizuki, who has been living in Japan for dozens of years and was one of the subjects of the film, presented a stereotypical example a Japanese American's experience in Japan; when they come to Japan, they are often ipressed with Japanese culture and feel comfortable with their identity, but after a year or so, they begin to notice some negative aspects of Japan and they often go back to the U.S. with the negative feelings. So most JAs in the States tend to talk about their bad memories. If they overcome that stage like him, they understand both positive and negative sides of Japan and look at Japan rather objectively.
I think the main reasons of JAs living in Japan are that they are satisfied with their life economically first, then they like Japanese food. Some of the subjects often referred to the food over here somehow for their choice of living in Japan.
We also have to take into consideration the age, though.