Two thoughts on why this might be intentional and not a bug. First, it could be designed that way, so the players can't draw the harpies into the camp as a way to cheese the fight and make it easier.
Second, Druids are nature lovers, right? And yeah, I know Harpies are magical beasts, not natural. But the Druids must know there's a nest right outside their sacred grove. They haven't bothered to clear it, so they must be okay with Harpies living nearby and they don't want them messed with.