I thought they spent most of the game offside at the ruck and breakdown, and it was barely policed at all. You only have to look at who is coaching them to understand where that comes from. The Wallabies of the early 2000s made a tactic of being offside. When it was finally (and correctly) policed in the 75th minute of this game, it came as a bit of a controversial "gotcha" moment.
However, to be fair, the All Blacks weren't actually compliant angels at the breakdown either, both with offside and with slowing the the ball down - sometimes legally, but sometimes not.
As to "that" offside call, its important to remember the Law as regards to when the ball is out of the ruck'
LAW 15: RUCK
ENDING A RUCK
17. When the ball has been clearly won by a team at the ruck, and is available to be played,
the referee calls "use it", after which the ball must be played away from the ruck within
five seconds. Sanction: Scrum.
18. The ruck ends and play continues when the ball leaves the ruck or when the ball in the
ruck is on or over the goal line.
Despite what you might hear from Justin Marshall (who does not know the Laws as well as he thinks he does), the ball is
not out of the ruck (and therefore, the ruck has not ended) when the scrum half puts his hands on the ball. The ruck ends when the ball is
lifted. Lawes was clearly at least a half-metre offside when the ruck ended, and that means he needed stop where he was until the ball was kicked or the scrum half ran 5m.
LAW 10 OFFSIDE
RETIRING FROM A RUCK, MAUL, SCRUM OR LINEOUT
8. A player who is offside at a ruck, maul, scrum or lineout remains offside, even after the
ruck, maul, scrum or lineout has ended.
9. The player can be put onside only if:
a. That player immediately retires behind the applicable offside line; or
b. An opposition player carries the ball five metres in any direction; or
c. An opposition player kicks the ball.
It was that half-metre advantage (illegally gained) that allowed him to charge the ball down and that directly resulted in the try being scored.
The Offside Law is one of the easiest Laws in rugby to comply with, just stay in an onside position and you can't be pinged for it. There is only one person responsible for the disallowng of that try.... Lawes himself.