This is an excellent example of how knowledge is passed on down through the generations and yet the original meaning and purpose behind that knowledge is lost. This is part of the mastery of this short story - we do not realise straight away that it is set in a post-nuclear disaster world and think that the setting is in a kind of stone age community with the kind of taboos and ignorance that we would expect from such a society. Of course, whilst this taboo was necessary for a long time, the story shows us how New York has become safe again over the centuries and John is able to travel there without danger of radioactive poisoning. It helps us to question taboos and also makes us realise that there is a time when they do not serve the interests of humanity any more.
John and his people have been forbidden for generations to go east to "the place of the gods." By the end of the story, however, we learn that "the place of the gods" is really New York City that had been destroyed long before in what seems to have been a nuclear war. John and his people are the descendants of the survivors of that war. When the war took place, the city would have been radioactive. Going into the city would have meant death from radiation sickness. Therefore, going into the city would have been forbidden at that time. As time passed, one generation after another, going east remained forbidden, but the original reason for its being forbidden had been lost.
it is forbidden to go east because they lie that the one who go there will die