I agree with you, they could have done it anywhere, and foam balls doesn't make it right. I get the anger, but I just doubt this was a deliberate attempt to make a point, more like a daft, unthinking decision.
I was outraged too when I first saw this, but it turns out that they are foam balls, so no damage would be done to the wall. That doesn't make it right, but it does add some context.
They just played for free kicks out wide to swing into the box. A cynical tactic, but I guess it worked for them. We look vulnerable to high balls in the box, but hey, at least we play sumptuous footy when we get the ball.