And the point the opposition is making is that putting it there without further elaboration or justification was a mistake, and quite frankly idiotic, and as the thread title puts it, poorly implemented. It would have been better served as a sidebar in in the class introduction with lore elaborations on why this taboo is a thing in the first place. I just had a talk over Discord on the r/dndnext server this morning on this very matter, and pretty much everybody agreed that this metal restriction is on flimsy ground at best.
And yes, I think I believe that blindly following rules "because they are so" without giving consideration as to why there are so is not a good thing to do. At least provide a setting justification or a balance concern re: their AC before you go about shutting down player options.
Heaven's to Betsy! I follow the rules! Oh noes!
Seriously - that's your big argument? You personally disagree with a particular rule so therefore everyone else must also disagree?
The setting justification is left up to the DM and the campaign. But I don't see this as being any different from any other rule in the book. We know how many spells and what level spell your PC can cast, if any, because of the rules. They don't need to justify why Fireball needs verbal, somatic and material components so they don't. This is no different.