Doubting Bro - I agree that it's easy for you, me, and anyone who has been around the WT to know that they should have such a duty. I think the trial judge had the same feeling, having been exposed to enough documents and testimony to know that something stinks.
The more difficult thing is setting out a general rule that is widely applicable, which is the job of the higher courts. Would the same duty apply to a Catholic parish with 1000 members, if it involved a victim and pedophile whose role in the church is merely attending services a few times a year?
What would larger churches that are not as hands on with members have to do to discharge the duty? Make an announcement from the pulpit upon even an allegation of abuse? That seems a little severe. Would contacting authorities be enough or do they have to do more? Should it depend on the level of control a religion has over its members? That could result in putting the church doctrines on trial in each case. These are tough questions - much harder than the obvious, which is to recognize that they have been dishonest, secretive, and acted in self-interest over the years with respect to prevention of child abuse.