In your fingers is a weapon, so you throw it. Maybe you're angry and you threw the weapon at a person you don't like. Maybe you're a pacifist and you threw it away so no one else could use it. Either way, when you have a weapon in your fingers you have to throw it.
You throw your weapon away because you're about to enter とうきょう and there are no weapons allowed there. There is, however, a big pile of thrown away weapons by the border, which is where you're throwing yours right now.