When I have been in Japan, the prices paid for equivalent quality and amounts of food are considerably higher than in the U.S. I would assume that those prices reflect higher wages paid to employees, so tipping would be considered a "bonus" to those workers. Tips are not "bonuses" to service employees here in the U.S. In those situations where tipping is customary in the U.S., employees are paid minimum wages and rely on tips to ensure reasonable compensation for their work. Whether you feel it is right or wrong, it is the custom and practice. And, having been in Japan several times over the past 20 years, I can say that I appreciate good service when received, but I have not always received better service than here in the U.S.