The description of this option says: "When this box is checked, this storygame will be visible (but not clickable) on your profile page. If unchecked, the only author(s) may view it and it will not be shown in your profile.". This turned out to be totally misleading.
I've enabled sneak preview just to show the story description in the profile, knowing that no one will see what's inside (it's a mess now).
But one can actually play an unpublished story by clicking on user points and even comment and rate it!
There was a test link to rate the story and one dude actually commented it and rated it 8 which is totally wrong. Now I've deleted the playable pages and links.
So you have to create 3 "dummy" stories to stop getting points for them and only the 4th story without points can have a sneak preview that can't be played.
Later I'll probably post the problem with a sneak preview post to bugs or wishing well, but it will probably not change anything...