Because in a lot of cases, *realism* that\'s provided with Pking and player thieving is unrealistic. Think about it, why don\'t you go out and stab that guy you really hate, that picked on you in highschool. Because it\'s illegal, and you would are gauranteed to get into some kind of sever trouble. The same goes for pickpocketing, shop lifting, etc. The *criminal* or *evil* element of any given society is fairly small. In places where serious law enforcement exist. In most MMOs it doesn\'t. Therefore stealing some n00bs equip is really easly low risk behaivior. You are not going to get into trouble. So you say, why not have a player run police force? Well, because that requires an exterme amount of dedication from your players so you would have mob rule. Which in most cases is very unfriendly, and does not encourage a good sense of community.
Why don\'t we have NPC\'s do the guard duty, because it\'s not really feasable. It just requires to much extra work to do law enforcement of some custom set of laws, for any given location and the overhead is usually not worth it. So things like not being able to PK in a city is usually a good rule and a good approximation of realism. In the wilderness some distance away from civilization PKing would be allowed without warning. Though even that would anger some people since they spend all this time (sometimes years) gathering equipment and item, when some powergamer just walks by, kills them, sells their equip for pennies and wanders happily off to his next adveture.
The other problem is, in MMOs there are simply to many *extraordinary* characters, (ie high level chars), which means what is difficult for the average joe, is easy for them. So again, a world like that becomes unfriendly towards new players. And really that\'s why PKing comes down to. It isn\'t greifer and anti-PKers, it\'s how many new player do you have, and how easy is it for them to have fun. Because in theory, all other players of roughly equal level can defend themselves. That\'s why in reallity we don\'t have constant death and theivery everywhere. Because a lot of the time the other person doesn\'t have a clear advantage.
In MMO\'s we simply don\'t have that kind of distribution, and we don\'t usually have a set of reprocutions for doing something that\'s generally frowned upon. So in most cases it\'s a really good idea to limit PKing, or simply eliminate it, since most of the time it simply detracts from gameplay, rp, and character developement.
