They're probably talking in one of squad, platoon, or outfit. With the fade out effect of proxy the people farthest from whoever is talking wouldn't be able to hear. Sorry to burst the bubble. Also, cross faction proxy chat would result in relentless trolling. The only instances of cross faction voip I've encountered were added as jokes specifically for the purpse of trolling. One was an L4D mod that allow hunters to yell at whoever they'd pounced, and the other is in BlackWake so you can yell piraty obscenities at each other while swashbuckling during boardings.
I meant, if it were available they could use local chat instead. Of course they are using their own chat (because global local chat does not exist).
