Players standing in an offside position during free kicks

In a FA Cup match Manchester United's Juan Mata scored a free kick goal versus Shrewsbury.  This goal should not have been allowed due to offside - interfering with an opponent. 

As you can see in the video,  three United players positioned themselves in an offside position directly in front of the keeper with the aim of blocking his view.  As Mata moves to take the free kick the trio attempt to get back onside but fail to do so.    The clear movement (obvious action) of the three players clearly impacts on the ability of, in this case, the goalkeeper, to play the ball.

    If an attacking player moves to an offside position for a clear tactical purpose, and then remains in an offside position when the ball is last touched/played by a team-mate, they run the risk of being penalized for interfering with an opponent.

Part of FIFA's definition of interfering with an opponent includes "clearly obstruct the line of vision of an opponent...making an obvious action which clearly impacts the ability of an opponent to play the ball." 
The word "impact" applies to an opponent’s ability (or potential) to play the ball and will
include situations where an opponent’s movement to play the ball is delayed, hindered or prevented by the offside player.

PRO offers the following guidance in this type of situation:
    Officiating crews should use preventive refereeing techniques when players take offside positions for tactical purposes on free kicks.   The assistant referee should inform the referee using the communication system that the players are in an offside position and the referee should inform those players that they run the risk of being called for offside even if the free kick is played directly towards the goal.  (examples include a shot on goal or a cross)
Players may choose to remain in an offside position and you still need to wait to see how the free kick is taken before a player may be penalized for offside.  If the offside positioned players move to an onside position before the ball is played by a team mate they would not be offside.