I felt they explained it well enough. Xenophobic druids decided to enact a ritual that would seal off the grove from outside forces. They'd been conducting the ritual for a few days and were nearing completion which meant that the Teiflings had to go. The Teiflings were reluctant to leave because the Goblin threat was still out there and if they tried to make their way to Baldur's Gate they'd be killed. Desperation on the druids part was never a factor as it was simply time running out for the completion of the ritual. There isn't really a choice between druids and Teiflings; it's more do you enact the isolationist plan for which you will gain nothing and makes sense for the situation; or do you solve the groves issues by one of the three other methods. The other three being: Kill off the druids, remove Kagha through force/persuasion, or go save Halsin/kill the puppet cult and let him deal with it.
It's apparently been stated by Larian's head that they had to force their writers to add in "evil" choices and those boil down to; be an idiot and kill someone that could've been useful down the line. Personally I prefer when the decisions are more grey and you can argue which was the better choice. I mean if they're going to go with a psychotic evil path they needed to have a psychotically fun companion to go with it, and they very much don't have that.
Also I have to agree with the lets not have every single thing that breaths be romanceable. That would result in either a space captain scenario, or a whole lot of shallow romances.
