Now I just thought of this, so maybe I'm wrong, but I don't think "I don't understand how someone can think X" is really meant as any sort of piece of reasonable logic, or a substitution for one. I suspect this is merely the sort of stuff people come up with when made to think about it.

Rather, "I don't understand how..." is an appeal to the built in expectation that things make obvious sense. If I want to claim that "what you're saying is nontribal and I have nothing to do with it", stating that you're not making sense to me works whether or not I can actually follow your reasoning. Since if you really were not making sense to me with minimum effort on my part, this would imply bad things about you and what you're saying. It's just a rejection that makes no sense if you think about it, but it's not meant to be thought about - it's really closer to "la la la I am not listening to you".

Am I making sense?