One could argue that they're desperate for any sort of help and so are willing to debase themselves somewhat for that. And they're tieflings, they know all about facing unfair racism so it doesn't surprise me that they're slower to cast similar judgement, they know better. Especially since I don't believe they've personally encountered drow before.
It also makes sense to me that Astarion wouldn't insult you to your face. We're the nominal leader and he's making at least a token effort to stay in our good graces.
No excuse with the duergar though.
