For me, it is simply justifying the ban/kick I give them when they grief. If not, they may come back later under a different name stating they didnt know they were griefing, thought it was ok, etc. And that would make me feel a little bad.
It's like tunnellers. I hate tunnels in my maps, yet people make them when they aren't informed so. I will ban if they disobey the rule - if I do tell them that rule. If I don't tell them, I can't very well ban them, seeing as they didnt know that the rule was there.
I guess it's a question of laziness versus how unfair you want to appear as an admin.
I can see where you're coming from, but to me it's not really worth it.
-Nongriefers think you're captain obvious
-Serious griefers don't care and are anticipating a ban should you catch them
-As for noob griefers who do it 'on accident', doesn't it make sense that you don't join a videogame and start killing what people made? I know I joined my first multiplayer server expecting to not be allowed to touch what other people made, with no prior experience (didn't even know what a griefer was). I mean I guess they need a chance to figure out what is/isn't acceptable, but really, if they decide to come wreck stuff I don't want them on my server.
But back on topic, I alwas tell people to not grief, not because I think they care, but to remind them that I am an op, I will ban if they griefs. If I open, they will notice that I am op, and respect me in some kind of way, because I can ban them.
But there seem to be little point in this, seeing as griefers will grief when there ARE people on the server that dislike grief. Also, making the griefer aware of your presence will make him wait till you are gone untill he starts griefing.