Doesn't mean they should have to accept you though. If a faction doesn't want you to join or wants to kill you you shouldn't be able to join them.
This is true, but then Larian need to show an explanatory cast scene and a few dialogues before the druids attack. So far, it looks like the player has not had the opportunity to be persistent enough to ask to become their friend, and they do not explain their hostility after the main character helped them.
