Why? Balance? I'm sure he didn't do it expecting the ball to be deflected up from his leg. If he even thought about it, maybe he thought it was less risky ("deliberate") than having his arms extended sideways.
I beg of you both to go outside and try recreating that movement with your arm at different angles, straight in the air guarantees you probably hit the deck after, so it isn't helping with balance.Are you stating that an arm above a certain angle is unnatural? What is the exact angle the arm can be taking into consideration physical dynamics? Straight? Ninety degrees and rigid?
The Dunk handball from a couple of weeks ago is more debatable, he's tucked in as much as physically possible whilst sliding, including bending his arm. But the hand couldn't have been more extended and did stop the ball continuing into the box