They don't need to be closed sessions there are a number of RP focused NWN PWs. You do need some sort of policing mechanism though and I'm not sure how you'd manage it on a commercial level as it would be resource intensive.
In theory that is true ... and I'm sure that many here have had sessions where everyone was behaving properly, but unless the policing mechanism you mention is more strict, flexible and permanent than what I've come across so far, it is only a matter of finding the loopholes for the rules to become meaningless.
Take a team based shooter, for instance:
1) An idiot starts shooting his own team.
Consequence: A kick voting system is implemented
2) Sore losers or just plain old idiots starts to call votes to kick good players on the opposing team (or just players with a better score than they do).
Consequence: Kick voting is replaced by a complaint system where you can file a complaint against a team kill. With enough complaints accumulated you get kicked off the server.
3) Idiots start to provoke you to kill them, run into your line of fire on purpose, jump on top of your thrown grenades, etc, etc, and then they file a complaint.
4) the list goes on and on and on ...
I'm sure that with certain niche genres you can actually successfully manage to pull off a proper session that is still basically open for all, but as soon as you're getting closer to the mainstream genres/ widely popular games, even policing servers only mean that you have some kind of means to get rid of the morons when they screw up your game, not to keep them from ruining your experience in the first place. Only full control over who gets to join can guarantee that (i.e. LAN or closed sessions).