ADnD Player's Handbook:
Unlike the cleric, the druid is allowed to use only "natural" armors -- padded, hide, or leather armor and wooden shields, including those with magical enhancements. All other armors are forbidden to him. His weapons are limited to club, sickle, dart, spear, dagger, scimitar, sling, and staff.
It's not metal weapons, but metal armors that're banned.
The thing about weapon is that Druids are pretty much limited to using things that resembles "farm tools" as weapons, even if those might have metal in them.
(It's also weird that they don't include scrythe, flail and nunchaku since those are farm tools as well... Okay so the main books don't have nunchaku and scrythe, and flail was kinda stretching it...)
Think of it as a (kinda
dumb 80s) way of portraying the "Druid = Nature" theme.