I would argue the politeness bit.
I wouldn't. Maybe you aren't aware of how much
need some people have to keep the peace, especially if they've been raised to be good guests or to not make a fuss.
There are people who have plant based diets and there are vegetarians which are different.
Since the definition of vegetarian is "a person who does not eat meat
: someone whose diet consists wholly of
vegetables, fruits, grains, nuts, and sometimes eggs or dairy products" I would think you're being unnecessarily pedantic here. You don't have to swear an oath to be a vegetarian, or even vegan.
If a Druid somehow accidentally put on metal armour nothing would happen. People are then going to take that and turn it into some gotcha that lets them wear it. And that isn't playing in good faith.
A lot of the "issues" that get brought up by various rules that I see online I've never seen in play because I've never played with someone who is trying to break the game.
Not everyone in this thread has done that. Some don't like the rule and that's fine.
For the record it is non-negotiable at a table I'm playing at. If that is a red flag so be it. Play a Nature Cleric. I want the classes to mean something.
"Playing in good faith" isn't a game rule, though. It is by definition
meta-game. There are plenty of people who game for other reasons, including power game-y ones. And while I would prefer gaming with people who
don't do this, who game according to the spirit of the class and not the letter, "you can't do this because I think it means you're a bad gamer" isn't a good enough reason to ban something.
Either have official game rules or make definitive homebrew rules that you are willing to say are homebrew and not just "of course it's like this".
Also, druids are
part of nature while nature clerics
control nature. There's a difference there--and why nobody blinks an eye if you say that druids gain their powers from the earth itself rather than from a god--and and metal is part of nature. You can decide that "nature clerics can't wear metal armor because the nature god says so" and that's perfectly fine. But the earth itself isn't talking and sending down divine edict in the form of holy writ, so there's no real reason why this proscription was invented in the first place.