If you allow cross faction chatting, it will cause rampant cheating.
How's that? Like warzone rigging or what? Because you could still probably set that up outside the game if you really wanted to, and if you were trying to set it up with randoms they'd never listen. "Okay guys, we're all going to lose this round. If you happen to spawn in with us next time, maybe then it will be our turn to win. It all depends on if my inquisitor friend gets enough experience." The randoms: "Screw that.".