You're also ignoring:
The Blackrock Orcs, the Dragonmaw Orcs
The Grimtotem tribe Tauren
The Amani Trolls
The Night Elves who follow Fandral Staghelm
etc etc, I can't think of more off top of my head.
For Humans there's tons of in-race fighting (Defias Brotherhood, Scarlet Crusade, and the aforementioned Kingdom of Alterac, the Freebooter KulTirans, and probably more I'm forgetting).
Point is there's a lot of in-fighting between members of the same race. This has been true since WoW released.
Horde and Alliance are 'ethnocentric' in the same sense that Sports teams are 'ethnocentric.'