I think the rules were made fairly clear and precise when Nigel Clough Gary Crosby err... "used his head" (headed the ball off the goalies palm as the keeper was about to drop it to boot it)
That is a very different case. You aren't allowed to interfere with eth goalkeeper releasing teh ball from hand and kicking it. But once the ball is on the ground, rolling, it is normal football.
The question here is whether what he did could fall under "unsportsmanlike conduct". Some people think he needed to ask permission from the ref before re-entering the field, but that is not the case when he has left the field as part of normal play, and the rules don't say you can't delay before re-entering.
Whether delaying to gain an advantage is unsportsmanlike conduct is down to the opinion of the ref, and in this case, he obviously didn't think it was.