An assumption is that people want their ingroup to have a positive image relative to outgroups, and so people want to differentiate their ingroup from relevant outgroups in a desirable way.  If an outgroup does not affect the ingroup’s image, then from an image preservation point of view, there is no point for the ingroup to be positively distinct from that outgroup. 
Some quotes have been edited and condensed for clarity.
How do we defend the claim that an item of reasoning should be labeled as a particular fallacy? A major goal in the field of informal logic is provide some criteria for each fallacy. Schwartz presents the challenge this way: