Druids: "Hey, this is our grove. You refugees can't be here because you're making things more dangerous for us."
Tieflings: "But we're refugees running for our lives. You have to let us stay. Hmm. Maybe if we got rid of Kagha."
Also Tiefliings: "Aradin, you can't come inside the grove because you're making things more dangerous for us."
Aradin: "But we're running for our lives!"
--So they're running for their lives, just like the tieflings. But they're expected to die nobly and get eaten by goblins because... they should be saving the tieflings. But the tieflings shouldn't be saving them. And the druids should let the tieflings stay because... they should be saving the tieflings. While the tieflings are considering getting rid of Kagha; in fact, they're open to paying someone to do it.
The difference is the urgency of the threat.
Aradin live in the Grove with the refugees. It's just that at the time they come pounding on the gate, they're literally being chased by goblins, the goblins that are trying to find the grove specifically to murder everyone inside.
Zevlor isn't against allowing Aradin back inside. He's against doing it when there's literally goblins 10ft behind them.
Aradin, like you, doesn't seem to grasp the danger that having goblins 10ft behind him entails. Besides the "We'll die if we confront them" aspect.
The Druids simply don't like that the presence of the refugees has made them a target for those such as the goblins (The fact that they continue to kick out the Tieflings even after you clear out the goblin camp is another poorly explained plot point) but the threat is not so urgent as to simply banish them from the grounds in an instant.
