I think that it is one of those understood rules. When a guy knows he is in control of the fight and his opponent is out, he purposely punches the mat. I'm sure it is because he doesn't want to inflict any unneeded damage on his oppenent. However, I remember one of Josh Koshchek's fights against some opponent-forget who-when Koshcheck had him knocked out, and this rule should have applied, but then still nailed him with 2 or 3 more brutal punches to the face. He is an asshole.