Many servers offer flying around in survival. Many players have begun using this to fly leashed animals around. The problem is that they bounce up and down, creating Portal-esque momentum generation. This causes the mob to die the moment it hits the ground, requiring you to land in water to prevent deaths. Even worse, the leash breaks all the time, causing more problems. In order to fix this, fall damage needs to be disabled for mobs currently or recently put on a leash.