Well if your earthbound and have a MV of 0 then you can't really turn around to fire on enemies. in the Great Grey case the limit to fire on enemies is already down to 90 degrees as well. And without the possibility to turn around while earthbound all you need to do is to get outside the weapon s arc of fire to get free shots at the flyer. Yes flyers ain't good as earthbound, but the possibility to land should still be there. Specially since the terrain effect sandstorm demands that all flyers take ground or dies. Else the rule simply could say, all flyers die.
