PvP gives you ways to punish undesirable behavior. Nothing suggests to someone that he may have violated community norms quite the way decaptiation does. Really open PvP lets you take that option to those nominally on your own side, like the guy who parked a mammoth on an event NPC or that other guy who is constantly spamming in town.
PvP also gives griefers far more opportunities for undesirable behavior. How much time are you willing to take away from your plans to police those who are just in it for the lulz? And MMOs reward nothing if not time: how are your levels and gear relative to this guy who obviously has far too much time on his hands?
In practice, PvP policing will take place only where griefing and ganking would be inconvenient for the top tier of characters. Level-capped enemies rampaging through the mid-level zones? Meh, they’ll get bored and leave before I can be bothered to get there. They’re killing the auctioneers? Oh, it’s on!
Seriously, who has time to stand around the newbie areas just in case someone decides to pick on them? That’s what we have NPCs for! If only we did not fight far away from the NPC guards…