A. Because the refs want to watch, too
B. Because you can't penalized a fight until it happens, or its just roughing (which sometimes happens)
C. Because they know that sometimes it is best to let a little steam out of the game
I think the police should step in. A fight is a fight, regardless of whether it started as a game or not. The exception being boxing, martial arts, of course.
The 'penalty' isn't treated as a traditional penalty. If you think about it, they don't give an advantage to either team, so no one is actually being penalized - assuming that all players are of equal value. To me, it's listed as a penalty because they need a reason to keep the two parties away from each to heal/avoid delaying the game.
Now we can discuss what actually defines a penalty.