To be fair we don't know the specifics (or at least I've not seen the specifics in any article).
If it's like the Aviva Premiership then the coaches are allowed to bring things to the referees attention but only throught the approved system (I think they're allowed to have three points, and only give them to the fourth official).
Saying "Why wasn't Hooper carded for the hit?" in the tunnel is a bit different to "I'LL KILL YOU, I'LL KILL YOUR WHOLE FAMILY". I think what may have caused this whole "scandal" is that the RFU film the tunnel pre-game, at half time, and post-game for youtube, so there's concrete evidence of them saying whatever they said. I'd be incredibly surprised if this kind of thing hasn't happened in other games before.