Um, this has nothing to do with keywords per se and everything to do with localization. Japan is a mobile-centric market. Their cellphones are designed around the native language. So in terms of speed and efficiency, it makes a lot more sense to do a search with Japanese characters than switch to typing out the romanized URL.

Think about if Japan and the Japanese language had become the dominant driver for the Internet. Would it be faster for you to search for "Apple" in a local English-based search engine or switch your cellphone to Japanese input mode and type out Japanese characters for what would be the Japanese URL of Apple Inc.?