Ah, so I see we're back to ignoring what the definition of a rule is, and making our own up again. And also completely ignoring all of the basis behind why that rule exists. No "rhyme or reason"? Dude, reread your own posts when you quoted that SA, because it tells you what the reason is right there. You seem to be under this huge impression that if you don't agree with something, then it never exists even when you're quoting it yourself. That takes some serious blinders, I'll give you that.
Yes, the Sage Advice provided additional clarification, but the book itself provided no clarification, which is what I was pointing at. People had to literally ask the game system developer what the heck that meant. The game developer simply assumed people would get it based on books from 1976 and 1978, which even that he was wrong on, because neither book even clarified it to be a taboo, and the latter example (original AD&D PHB) made it a mechanical limitation that spoiled their magic as opposed to just a choice they made. This is ignoring the fact that it was not taboo in 3E or 4E, with 3E spoiling their magic, and 4E having no penalty at all minus not being proficient, which could be negated by becoming proficient. The only edition that was different from this was 2E, which stated they may only wear "natural" armor, but it didn't specify any reason as to why that was the case, nor did it make any mention of metal or explain what "natural" meant. The 3E PHB released in 2000, so Druids had mechanics related to wearing metal armor for 14 years of D&D before 5E ever released, which is why it threw people off to see such a poorly implemented line without any sort of hard or soft rules behind it.
Even now with the Sage Advice clarification, people seem to be willfully ignoring the parts of the Sage Advice where it's said that not wearing metal is simply a preference of Druids, that Druids
can wear metal armor, that nothing in the game system prevents them from doing so, that there is no penalty, and that they are proficient in its use. That has all of the elements of being a choice, not a hard restriction that cannot be bypassed if the character has different beliefs, much like a Cleric and their deity, or a Paladin and their oath, or a Warlock and their pact, or a Monk and their monastic traditions.