Osaka trip: unexpected carry on luggage

When leaving Osaka, there was a display at the airport showing you what things you are not allowed to take with you on board the plane. Among other things, you are not allowed to have ninja throwing stars (shuriken) with you.

Japan, the land of ninjas, where bringing throwing stars into the airplane is a common enough problem to warrant a display case in the airport.

First I thought, "How much of a problem can this be? Who has shuriken with them when they travel!?" But then I figured that it is probably common that tourists by throwing stars as souvenirs and leave via the international airport.