It's a bit weak IMO. But I can understand that some people should be fine with that.
And there might be reasons, more than we can understand, to why players choose to go to the "dark side". Who am I to judge? Also, I consider "Open" as the open forum of free speech in society. I dislike and despise what some people say and think, but having free speech is more important than to silence their voices.
In Open, if griefing was cracked down on, you bet normal, healthy, non-evil kind'a PvP will suffer too as a result, and in the end, no griefing will lead to no PvP combat at all. We all will be forced to "get along". Piracy dead, unless it's against NPCs. Combat, only with NPCs. Any shooting another player, insta-death and prison for million years. Fine, griefing is gone. But so is a large portion of all other play.
With that said, Open being fine for griefing in my book, PGs that have specifically said you're not allowed to do it, that's another issue, because it's a breach of the "contract" by the player, not the avatar/character/role-play, and that shows a person's intent reaches beyond just role-play.
--edit
Sorry for the hyberbolic rant.

I just feel they fit for a part in the game that is interesting and challenging, and wouldn't want it any other way.