If friends and foes all wore labels to define certain aspects about them which we attribute to each status, decisions of this type would be easier.

In the examples of opposing teams and gangs, this is somewhat the case, the sides are defined, the reasons for friend or foe already agreed upon by each party of allies. No further evaluation need be made, because the labels designate friend or foe according to common agreement.

In regular life, labels and stereotypes don't always provide enough information to decide whether someone is friend or foe. Yet, people make snap judgements based on body language, instinct, actions, words and other criteria, as to whether someone is a threat or a non-threat(loosely speaking-friend or foe).

I think much confusion has come out of casual use of the terms "friend" and "enemy". When in reality, what is actually being defined is much more simple and less related to relationship and more to survival instinct.