We have alot of non-native things because they were brought here by non-natives. The Heavensturn helms, all provided by Far Eastern tourists. Yukata, brought in by globe-trotting traders who sampled flavors of the Far East. Fuma... not sure, there's no telling how it got inside an ancient, unexplored dungeon long buried till the Calamity. Ninja is none of these things. Ninja is a discipline, brought over by a Far Eastern immigrant, who is the only one outside of certain NPCs who knows the abilities and arms of said discipline. For random vendors to have Japanese daggers would require a larger population of Ninja than what we have, who are also shameless enough to capitalize on the knowledge and equipment therein. From what I gather, this is not the case.