The counter argument is that goalkeepers don't want to have players allowed to stand right in front of them when trying to see the ball being headed and to set themselves for a save. They often take a small step forwards before diving and that would get in the way of that.
I can see both sides of it. Yes on this occasion Fabianski could just about see the ball but having a player a foot in front of him no doubt could affect his decision making.
The counter counter argument is that, is Chirewa really interfering with his ability to save it? On the corner Ward-Prose scored from Antonio was all up in Sa's grill trying to block and disrupt him. Antonio wasn't offside but interference on the GK is fine in that instance and not the other, just because of where the ball is coming from?