Acceptance of sweet rice dishes in Japan
Mochi is a sticky rice cake that can be served both sweet and savory.  Sweetened rice pudding is not an uncommon dessert in Japanese cuisine.  I think your original premise, that sweetened rice is unacceptable in Japanese cuisine is flawed.  Rice is a staple in Japanese cuisine, and is used in numerous ways.  Sweet preparations are certainly common.  Perhaps your father's story, and your anime example, illustrate better what might happen when people expect one thing, and are presented with another.  However, I think your generalized conclusion is incorrect.