Also noteworthy within the lore of the world there have been increasing numbers of good aligned Drow thanks to the actions of Drizzt Do'Urden. (Edit: Far from mainstream, but established as not impossible)
notcreative put it really well. There is a big difference between the baser, more monstrous of the monster races like Gnolls and the smarter monstrous races like Drow and Hobgoblins (remember Hobgoblins get an Int bonus in Volos).
I agree, they are too gullible - they should be more mistrustful and warily.
But their situation is desperate and they need every support, they can find. I think it's realistic, that they wouldn't be hostile, if they see, a drow or a goblinoid is slaughtering their enemys, without gaining an obvious advantage from it.
Edit: Goblinoids or Drows are no worgs or gnolls, and i assume that tieflings know the difference between them and monsters
On the topic of places attacking monstrous races immediately, I think there are another couple of factors to keep in mind. I think a shadier place like Baldur's Gate would be much more likely to let certain races in than someplace like Candlekeep. And depending on how various DM's and players choose to manifest the 'good' of good aligned places/characters, I can imagine coming to think something like 'I won't throw the first punch but I'll keep a close eye on them' as a logical conclusion when dealing with a monstrous race that is behaving in a good manner.
