I'm in NEW England (US). There may be a cultural difference, though I have refereed U-19 matches here and you do have a point. There seems to be more flare-up potential among the younger guys, perhaps due to the combo of immaturity and testosterone. The action described above sounds like the victim had some peculiar weakness or the perpetrator got in an unfortunately lucky blow, perhaps to the back of the head or neck. All I can say is that, given the inherent violence of the game, there are damn few serious injuries and this is the first death I've heard of. Every year or so, there'll be a spinal injury, usually from a scrum collapse, but the laws and officiating training have cut down on a lot of that and the laws for U-19's are very safety oriented.
Look, people choose to play these rough sports. I have no patience for the "Ooh, they're so rough, this should be banned because one person got hurt!" crowd.