I don't think there is any specific justification for this in the code. I believe that it is however a popular solution to insulate the Bare conductor and land it on the Neutral bar, making sure that some unbroken sheathing extends through the clamp/connector into the Panelboard. This way it will not raise the potential of any exposed metallic parts.
It's a good compromise in my opinion. I'd say it's much better than allowing 3 wire service to remote panelboards because they are existing. (This has been discussed before). Perhaps you can check at IAEI BB to see what other AHJs' are allowing under these circumstances.
[This message has been edited by Bill Addiss (edited 05-24-2001).]