Dude they're Orcs.
They live in a society where that kind of treachery and violence is commonplace.
I mean, if the DM pulled that in reverse to you (the Orcs seem friendly guys... BAM SURPRISE!) wouldnt you argue that there was no way in hell your PC would be trusting Orcs, let alone ones you've only just met?
If no sane PC would trust a bunch of Orcs, why are the Orcs trusting anyone else? They live with deception, murder and treachery as a daily freaking occurrence!