[mad=dog]

In basic D&D the rule is as simple as you put it - no metal allowed at all.

Before me move on we have to remember that the names put on things are names of convinience. They do not speak English in Amn and would most likely not use the word "scimitar" at all. We have simply adopted some real life names of familiarity to assist us in description. So druid does not mean British druid, but "anyone who worships and draws power from nature".

Sadly the rules are not explicit on this point. They hardly ever are in AD&D. You have to figure out the reasoning yourself. The weapons allowed are Club, Dart, Spear, Sickle, Dagger, Scimitar, Sling and Staff. However there is no mention as to why these weapons are chosen over others. The Complete Druids Handbook does not provide any clarification either.

My reasoning is that a sickle is a sacred blade to AD&D druids. I quote from the Ethos paragraph
"Mistletoe is an important holy symbol to druids and it is a necessary part of some spells (those requiring a holy symbol). To be fully effective, the mistletoe must be gathered by light of the full moon using a golden or silver sickle specially made for the purpose."

However this does not account for scimitars. Thus I have always thought it to be reasonable to adopt the "any crescent blade is allowed, but any straight blade is forbidden" rule. When you add a new weapon to the list you have to decide if each class dependant subset should include or exclude the weapon in question.

(If you say that a druid could just use a curved longsword, I'd say that any curved longsword would be termed a scimitar)