"I see the druid restrictions as something along those lines. Notice that they will freely use metal tools and weapons. They might lack proficiency in some, but are given others like the scimitar... So I don't see it as any kind of 'vegan' analogy unless you know many that choose to be vegans for ethical reasons and yet say still wear fur!"
-James
I think the reason that they can use metal weapons and tools is simply because the "stone age" versions of those items come with some heavy penalties.
The stone weapons for example out of 3.5 were give a -4 to hit and damage, wether or not you knew how to use them. It was said that it was a quality issue. I think to balance it out they simple allowed the Druid class to use metal weapons.
As to the subject at hand I would agree that a druid who dresses up his pet in metal is in bad taste and doesn't really fit the "spirit" of the Druid class. That being said, if they want to do it I would say let them, or come up with hide barding or something similar so they wont be mocked by the druid community.