In Rugby, head to shoulder contact is a red even if its accidental. They give no room for interpretation because they consider the blow to the head that serious. And yes you're probably right, the frequency that it happens in Rugby has almost certainly forced them to bring it in sooner.
That's not true at all. I am a rugby ref and there is plenty of room for interpretation. Most notably if the tackler goes low and the ball carrier ducks into the challenge, that's a clear mitigating circumstance - in fact, if the ball carrier is too low, they could be the one penalised. Or the instance I saw in a game a week ago, where the tackler was pushed into position by his fellow player, that wasn't deemed a red card either. Or it could be that the tackler slipped and while falling, struck the ball carrier. Or if it's not direct contact but slips up to hit the head.The protocol these days is to award a yellow and see if there are any mitigating circumstances