Ugh. You're right; guidelines like that need to be clear. Subjective qualities such as "artistic merit" still end up getting tested in court-- one doesn't have to be pre-law to know that. What a mess. Still, if I had a twincest fanfic community, I wouldn't be surprised if the mods deleted it upon discovery, even without a clear policy against its contents. It would piss me off, but it would also be understandable. That example doesn't cover LJ's deletions, I realize; I was merely defending their wish to avoid hosting pedophilia, not the poor way that they handled it.