They dont actually allow Slash at ff.net. They just don't check the stories. People that before wrote their stories as NC-17 now write exactly the same, but give it a lower rating when they post them. For some reason the ones who write Slash seem to be the ones that do this the most. Go figure.
Anyway FF.net should get off their asses and check the stories more. If they want to have a rule against NC-17 stuff, they should damned well oversee that the writers keep with the rules. Otherwise it becomes just plain stupid.