The people that cry spy are the people that are hopeless in fighters, and are average at best in everything else. Their use of the horde makes them self-delusional as to their own skill level. Who cares about these people in game?
It seems like quite a stretch to connect those dots, but you are entitled to your opinion.
The fact is, this kind of "home team" mentality is engrained in us from a very young age. Sporting industries rely on this mentality to survive - as do governments. Remember school spirit? Somehow your school was supposed to be better than the other neighboring schools? Online video games are a microcosm of real life social dynamics. It's a natural extension.
Of course, I don't care what side I fight for. The only reason I don't change teams often is because I think it would be embarrassing to get shot down by people that I normally fly with! I know this too is a strange social behavior...