Kicking It Out For Injuries
Many of the unwritten rules are ‘gentlemen’s agreements’ of some sort designed to not embarrass or even help out your opponents. The top example of an unwritten rule to give shelter to your enemy is in soccer. If a player gets injured and the whistle is not blown for a stoppage, it is the duty of the player with the ball to kick it out of bounds so the injured player can be attended to, especially if they are on the other team.
Once the injured player is removed and play resumes, it is equally universally accepted that the team throwing the ball in will give it right back to the team who stopped play by kicking it out of bounds. This practice of fair play ensures that injured players can be out of harm's way and that neither team will suffer as a result; and it’s the accepted norm across the globe at every level of play.