Well, I don't know any of the details involved in their mission thing, but... They could've been told something like, "these supplies are food, please deliver them here!" Only to discover they'd delivered weapons instead!

Or maybe they were told, "these weapons are for X faction, but they're bad, so delver them to Y faction instead because they're the good guys!" And been tricked that way! ...Though, I guess that's still pretty bad...

[Sanzang crosses her arms over her chest, tilting her head from side to side as she considers.] It's not likely a terrorist group would come right out and say, "we're a terrorist group, give us these weapons!" And I don't see anyone just going, "oh, okay!" and handing them over just like that...

At least, I'd like to believe in our friends a little more, that they wouldn't do something like that knowingly...