It wasn't the wrong ruling. The crap Rabs and Gus were going on with that you can't run behind your own player, end of story, is wrong. Never was correct. Just because they score a try from it doesn't mean the "running behind the player" gained an advantage. In this case the advantage was gained because the Blues players stopped EXPECTING a penalty.
Scott didn't stop expecting a penalty, he stopped in anticipation of Hannant receiving the ball, this cost him forward momentum and at least 3 mtrs sideways. Below are the refs guidelines which were used for the interpretation of the try.
IMO the advantage gained by Qld was the fact that Scott had to stop then try to catch up to where he would have been had Hannant not been there which brings section (b) into play. It shouldn't get to section (d) IMO, but when you read all the guidelines you can see why the refs get confused.
a) It is the responsibility of the decoy runner/s not to interfere with the defending team.
b) The ball runner cannot run behind his own team and gain an advantage.
c) A sweep player may receive the ball on the inside of a block runner as long as there
is depth on the pass to him. It there is no depth he needs to receive the ball on the
outside of the block runner.
d) Defensive decisions that commit defenders to decoy runners will not be considered
obstruction.
e) Attacking players who loiter next to the play the ball can be interpreted as
obstructing the defending team.
f) In the process of scoring a try an attacking player dives through or into the legs of
the player who has played the ball a penalty will be awarded to the defending team.
This action will be interpreted as obstruction.
g) If in the opinion of the referee/video referee the play had no effect on the
scoring of the try the try will be awarded.