OK - so "If a substitute, substituted or sent off player, player temporarily off the field of play or team official throws or kicks an object onto the field of play and it interferes with play, an opponent or match official, play is restarted with a direct free kick (or penalty kick) where the object interfered with play or struck or would have struck the opponent, match official or the ball. " So there is no law against throwing or kicking an object onto the field if it doesn't interfere with play.... And there is no law against another player kicking that object or any foreign object at an opposing player, substitute, substituted or sent off player, or team official, match official or the ball.