This shoulder charge business is ridiculous :(
There should be a differentiation between a defender "bracing themselves" and an obvious dog shot from a defender.
The situation where a man with 30+ kilos on you charging your way and getting caught out (naturally protecting yourself as anyone would do as an automatic reaction), should be treated with common sense and not just a black and white definition of "shoulder charge" when compared to the sort of stuff karmichael or Inglis used to do on defenders. When there's an opportunity to make a regulated tackle, you should.
The issue is, how do you make it as black and white as possible to account for poor interpretation like the old obstruction rule had last year and the year before