Firstly, a burger is not poisen anymore then a candy bar.
Secondly, the Japanese diet is determined by several centuries of religious vegetarism and it's status as an island nation with the richest fishing beds in the world. You cannot say they watch their diet since in places like Tokyo it is not uncommon for people to eat out 3 meals a day, usually "fast food". It's just that their fast food is different from fast food found on continents with large rangeland.
... But I would not want to eat rice and fish everyday either.