So, I suspect this is just something built into the engine.
I've been playing around with adding UAV operators to some of my missions to see how useful they are. Let's say there's a place on the map, a nice, flat area with no cover. Let's call it Position Grayskull.
So the Human controlled UAV operator is set in a Terminal Humvee 5km away from Position Grayskull. Two squads of AI, one Blufor, one Opfor, are heading towards Position Grayskull. They are in the normal combat mode that the AI spawns in.
Now the UAV Operator has a UAV orbiting 500m up at a 500m radius. He's watching Position Grayskull. The UAV is not human controlled. The only way that the UAV Controller is getting info is from a camera attached to the UAV.
In tests I've noticed that the AI never attack each other. They'll even sit next to each other, happily crouched and "eating smores and painting henna patterns on each other", to paraphrase Ferrard.
When I play as a human and move from a position 1km away from Position Grayskull and move towards Position Grayskull I find that when I approach the area where the two squads are that I'll suddenly hear an explosion of gunfire as the two squads suddenly realize "Oh, shit! We're enemies!" Or so I suspect.
TL;DR: Is there any way to make enemy AI squads recognize and engage enemy AI squads at longer ranges?