I can see where you're coming from, some ref's could have gave a yellow for that, especially with how conservative the refs have been going the last couple of years. There really is no consistency in ref's these days.
The real question is, do you actually want a yellow given for that tackle?
My answer is no! This is rugby, a contact sport, he did at least try and attempt an arm, there was nothing in that tackle which screamed misconduct or intent to be dangerous.
I hope that similar tackles will continue to give the benefit of the doubt to the tackler because it is very very rare that players are going out there trying to drop other players on their head or blatantly shoulder charging. Now contact to the head is probably a different story too.