Why indeed? However they do mention them (PHB, 57: "whether those creatures are hostile toward you or not").Why would combat rules mention anything that's not an ally or an enemy.
Saying the rules say no is the same thing as citing a republican-democrat debate and claiming everyone is either a republican or a democrat. Why would combat rules mention anything that's not an ally or an enemy. Its so rarely an issue it's not worth mentioning.
I honestly didn't believe that there were rules on what enemies/allies were. That rule changes the whole feel of this game imo. Everyone who isn't fighting with you against whatever you are fighting is an enemy.
Thats... quite dogmatic.