But hasn't Yosef already forgiven them in the previous parshah? "And now, be not distressed, nor reproach yourselves for having sold me here, for it was to be a provider that G-d sent me ahead of you..." (45:5)
Perhaps this forgiveness didn't register -- they had been quite in shock from Yosef's revealing of himself.
Or maybe neither forgiveness, nor G-d's providence, just isn't part of their language.
So, having done evil, they expect evil in return.
After all, isn't the other is a mirror of myself?