Noticed there was a lot of confusion what a "Justice System" entails. Quite simply we will refer to EVE Online. In Empire space, when you perform an unlawful action, you can be attacked by local authorities (Concord). So even though the game is techinically free-for-all, it is also lawful. Your attacker incurs a risk in order to get their reward (loot from your cargo).
Quite simply, that is all it is. Placing an all-seeing government body within the game to enforce order. Think of 'cops' as a direct extension of the developer- enforcing order in a choatic land.
In EVE Online, the security systems go from 1.0 (safe) to 0.0. The lower the security, I believe the longer it takes cops to arrive at the location where you are attacked. 0.4 and below Concord will not help you. In high sec, it might be possible for pirates to suicide bomb your transport ship if they scan you for valuable ores. Make no mistake, EVE Online is FFA PVP however it just takes a bit more imagination to be a pirate in high security space.
So, we kind of arrive at the best of both worlds you see. The victim is pretty safe in high secuirity space due to presence of local law. Any player can still attack but they will have to incur the wrath of Concord.
Also, trying to smuggle illegal goods can get you in trouble with Concord. Stealing someone's NPC loot will flag you as freely attackable in high sec.
In World of Warcraft their safe zones make hostile action totally impossible. So, if a friendly walks up and steals your resource node nothing you can do. In EVE Online, they will possibly be flagged as hostile and you can attack without interference from Concord. You can also declare all-out Wars against enemy Guilds making them open to attack ANYWHERE
EVE Online is truly FFA however, you can still peacefully get around in high security space.
Anyway that's a quick rundown for you guys. As you can see, this concept can be transferred to any other type of MMORPG quite easily. Most mmorpgs already have 'guards' anyway but they lack this level of depth