Its for the Blue Knights... that way they cant hunt each other down
.
Ok, seriously now. If there wasnt a killshooter in effect, the Main Arena would be a Zoo, with new players inadvertantly shooting friends (green or orange icon) cause they think its the enemy. Or players that want to annoy another would just keep shooting him on takeoff..... the possibilities are variable and endless, the outcome pretty predictable.
A lengthy discussion on the subject can be found
here.