I think that the prevalence of JCB makes DCC not worthwhile in Japan. Furthermore, Japan is a by-the-book culture, so you know that DCC would always be presented as a choice if it were enabled. For instance, I remember a cashier at a department store explaining the return policy to me - I had to go get one of my Japanese classmates to translate - because the department store policy was to inform customers. It was obvious that I didn't understand. How many other places in the world would a cashier bother to do something like that?