+1
I feel like this is an oversight because they're merely business partners, not actual members of the faction. So they should not be hostile, especially after befriending the Zhentarim up north. If anything there should also be a secret phrase similar to what Rugan tells us, to make them stand down or even join us in the fight against the gobos.
Abdirak for example, even though he is in the gobo camp, does not belong to the goblin faction and leaves as soon as fighting starts. So the same should be for the Zhent traders. They should be neutral.