They could claim they aquired the codes from the black market etc, or lane hackers etc.
I say let Liberty enforce their rules on their system. If they want to kill all intruders, let them. It is their space. As long as it is within the server rules, they are perfectly entitled to create and uphold the laws of their systems. Considering it is a military zone, I doubt many would be allowed in. Only difference to RL is that in RL you will be arrested, hard to arrest someone in space where they can run away.
EDIT: Speaking of Zone 21, what happened to the other 20?