Based on the clues in "By the Waters of Babylon," what can readers conclude about how the rituals surrounding touching the metal developed?

Throughout much of the story, the reader has to use
inference, or educated guessing through the use of context clues, to figure out exactly
why John's tribe behaves in the many ways that it does.  To us, people who never touch
any piece of metal might seem strange and overly superstitious.  However, if you read
carefully, you can look at the following clues, and start to piece together an idea of
why his tribe has developed this practice.  The first clue is that John refers to "the
great burning" that destroyed most people on the planet.  If you think about that, it
would have to be a very, very large event to wipe out most people.   Possibilities? 
Meteor hitting earth, or a nuclear bomb.  If it was nuclear bomb, there would be a lot
of leftover radiation.  Next clue:  John refers to Dead Places, where everything is
dead, including the people that lived there.  If you combine that clue with place that
they aren't allowed to go, namely a City, then you can hone in more closely on the
nuclear bomb scenario--bombs are normally dropped on cities, and wipe out the city and
everything surrounding them for a while.  That would make everything in its radius
"dead".

So, piecing together those clues, and assuming that
it was a nuclear bomb that took out the cities and killed everyone, then you can assume
that the metal products contained radiation levels for a very, very long time.  Imagine
the first survivors foraging for supplies and handling the metal; they would have gotten
sick from radiation poisoning and died.  The people would have passed that knowledge
down from generation to generation, and eventually, it would become one of their rules. 
The reason why might be lost, but the rule is still followed.  I hope that those
thoughts help a bit; good luck!