its he tho who has put his arms into a position where he can block the ball with them
Why did he not just stay facing the ball where he was when it was struck? That would be defending
Altering your body shape into a position where you can now block the ball with your arm should be penalised.
He had only one intention when he by his own free will changed his stance, to block that shot by any means possible.
Defender has a head,chest, shoulder? To defend with here. He opts for use of the arm
When somebody is setting up to hit a ball towards you, you naturally go side on. The instinct to protect yourself comes before the instinct to keep your hands away. You never see defenders not go side on when blocking shots, its unnatural to do that.