The purpose of localization, when applied to fictional works, is to transcribe a foreign story while preserving the intended message/evoked feelings and not confusing the target audience. If your audience is a bunch of western kids that have no idea what "onigiri" is, you may opt to switch it to a snack they would be more familiar with. Like donuts.
This is the exact same reason why, for instance, all the holidays in Animal Crossing get changed from Japanese holidays (White Day, Doll's Festival, Golden Week, etc.) to western ones (Halloween, Easter, etc.) when it is localized.