If you duck into a tackle you generally don't have an opportunity to break the tackle.
Whereas the shrug is actually quite effective at breaking tackles to continue play (Selwood actually does it heaps), it should be considered a legitimate option for the player being tackled and the onus is on the tackler to keep the tackle lower.
I don't think anyone's calling for the shrug and duck move to be outlawed, just that it shouldn't be rewarded with a free kick.