What has bending his arm out of the socket got to do with anything?you have to consider it’s not humanely possible to bend his arm beyond his socket to hide it behind his back.
It’s not a clear penalty as you seems to insist.
It is humanely possible not to move towards a ball and put said arm in the way of the ball. If you can get across to block the ball legally then fine. If you can only get across enough to then have to make contact with your arm then arguably that is not fine. That's the chance he took and it didn't come off.
At worst a contentious decision. But for me a perfectly understandable one - there are plenty of circumstances with how the laws are written where a degree of personal interpretation is required. This is one of them.