I don't want to believe that this is a stupid 'political correctness' issue between two friends.

With nothing from the U of O, that appears to be the best guess around, though.

The snowflakes at Duck U would have had a cow in any barracks in the US Army when the Blacks started on the Rednecks and the honkies started Doing the Dozens!

And that's in the all-volunteer-ever-so-trained-to-be-respectful Army of  Iraq/Afghanistan mandated by civilian authority over the last 30 years or so, not the US military who fought in Viet Nam!

I was in both groups; at the platoon/squad/fire team  level in particular, there are seriously rude comments exchanged. 

Think tribalism.  Arabs have it fairly well-defined:  "I and my brother against my cousin, I and my cousin against the stranger."

'Brothers' exist at the squad level, especially in combat units. 

(Officers  do that sort of thing after spending some time together at the Club where everyone has selective myopia and selective hearing.)