I've had the impression that the tieflings are trying to manage their life as best as they can, not that they are "earning their keep". For example, I am not sure if the smith running his forge/shop there is all that beneficial to the grove. But what I find amusing is that what they have in common with the druids is that both ask you to undertake a mission for them, but neither is really willing to support you.
Nettie is a healer, surely she could equip you with some potions before sending you off. Zevlor could ask the smith to equip you with best weapons and armor possible. He could send one of the tieflings who knows the area with you, after all you are new there. Neither of them do, which just leaves the impression that neither is all that invested in the survival of your party. In the case of Nettie, considering she knows you have a mindflayer tadpole, perhaps she considers either you succeeding or failing is a solution to the tadpole problem. With Zevlor, you are likely just another mercenary band to him. If you succeed, great, if you don't, no loss for him.
Nettie is at least helping you. So you are obligated to help her back. But of course taking down an entire Goblin camp - or even just sneaking in and rescuing Halsin - is a bit much to ask. In the end, you do it because he is the only one who can help you with the tadpole.
