It seems to me that the same would apply to any in-group. The reasoning runs more-or-less as follows:

It is us (not me personally, but a group with which I strongly identify) that is treating this person badly; since we are doing it, then he must deserve it. Since he deserves it, he must be guilty. This is because if he did not deserve it, then I would be horrified at the actions of people I have always tried to emulate; and that, in turn, would mean that I had already given some support to an evil group, and had indeed put some significant effort into being a part of that group, taking up the group norms.

If the group is evil, or does evil actions, then I am evil by association.

And a good person does not want to reach that conclusion; therefore, the person being punished must be guilty. And thus, good people do evil things by not acknowledging evil being done in their name as what it is.