Of course, brothers and sisters, and good friends joke around and say mean things. @Robert Lindsay has noted that. However, out-group stuff isn’t cool. It’s not busting someone’s balls but rather grounds for a fight. I mean even the slightest disrespect from an out-group can cause Hell in a school, prison, or work environment.
Now, why is there the difference? You tell me.
Note: Sometimes blogs and forums can be an in-group, and even if an out-group bugs you, it’s often so distant it seems comical (like the insults hurled at people on Stormfront).