Technically, there is nothing wrong with the goal, nor
the actions of the referee Martin Hansson. There has clearly been a
transgression of law with Sol Campbell's backpass and, once the
referee has awarded the free-kick, the rule is that advantage is
immediately given back to the team who have been
wronged.
In the spirit of the law, it can be argued that the defending
team should be given an opportunity to regroup before the kick is
taken, but that responsibility falls to the players.
Fabianski could have held on to the ball or thrown it away - as
we see so often - thus risking a yellow card but ensuring that
Porto would not be able to take a quick free-kick. As for the role
of the Porto players, how many times did we see Thierry Henry take
a quick free-kick during his Arsenal days under Arsene Wenger?
Most memorably, in an FA Cup tie against Chelsea - and we didn't
hear his manager complaining then.It could perhaps be argued that
Hansson's positioning prevents Sol Campbell from making an attempt
to get back, but that doesn't make the referee wrong.
West Ham and Chelsea fans will remember a similar situation
involving myself in a Premier League match some years ago, when I
was standing between the ball and West Ham's goal, and then stepped
aside to allow Jimmy Floyd Hasselbaink to take a quick free-kick,
which flew past David James into the net. Looking back at it, I
admit that my positioning could have been better but, in this case,
Hansson hasn't blocked the view of the Arsenal goalkeeper and, by,
asking for Fabianski to hand the ball back, cannot be accused of
taking the Gunners by surprise.

1. Sol Campbell touches the ball back to his
goalkeeper. Fabianski instinctively picks the ball up, leaving
referee Hansson no alternative but to award an indirect
free-kick
2. Ruben Micael attempts to wrestle the ball from
Fabianski, who initially holds on to it, but as Hansson arrives at
the scene of the offence, the keeper tosses the ball to the
referee
3. Fabianski turns his back to retreat to his goal,
while at the same time appearing to protest his innocence to the
assistant on the far side. The goalkeeper must be aware that,
having handed over the ball, Porto are permitted to take their
free-kick immediately, which Ruben does.
4. Ruben places the ball and passes square to the
onrushing Falcao. Sol Campbell, spotting the danger, makes a late
attempt to get behind the ball but is blocked by the referee.
Arsenal’s other defenders still have not retreated behind the ball
and Falcao has a clear run at the ball and shot on goal.
5. Falcao strokes the ball into the net just as
Fabianski finally turns to face the ball. Of the Arsenal defenders,
only Vermaelen reacts in an attempt to prevent Porto scoring. The
referee is confronted by Arsenal players and Arsene Wenger but he
is unmoved by the protest and allows the goal to stand.