I've never been able to figure this one since 1st edition. The druid is pretty well celtic themed, and most of their weapn proficiencies are traditional celtic weapons, eg spear, sling, club, but the scimitar is not at all associated with the celts. In fact one of the iconic weapons of the celts was the straight longsword, which is not on the list.
At first I thought maybe the druid's scimitar proficiency was supposed to represent the druidic sickle, but no, "sickle" is a weapon on the list all it's own. Then I thought maybe it was supposed to represent a machete for cutting through the forest, but a) that doesn't seem a very druidic way of navigating the woods, b) I would think a machete was a short sword rather than a scimitar, and c) machetes aren't exactly celtic either.
So yeah. Not really the most pressing question, but why DO druids have scimitar proficiency?
Any one have an answer for this perplexing mystery?