Not surprising, since Japan’s government considers all weapons anathema. Guns, swords, and anything you could actually use to defend yourself are banned (for the law abiding, anyway).
So, if you’re going to Japan and you own a tactical pen, leave it at home (and your pocketknives and guns and everything else that they might think is primarily a weapon).
H/T Sankaku Complex (NSFW. Very NSFW.)