The other apps are clients. The apps themselves don't actually contain any content, they're just code. An app that itself contains an offline copy of a book with NSFW text is not the same thing.
Meanwhile Reddit is a doubly poor example because even though the service contains NSFW content, it marks it as such, and then the client not only doesn't itself contain it but gives the user a separate opportunity to select against it when using the app to download pages.
