this has been mentioned in a few responses, but i think being drunk is a big part of it. north americans seem to go to these events or watch them with friends and its almost like a party atmosphere. ive never been to japan so i dont know if the same is true, but i dont think so.
north americans watch fighting, japanese watch a competition.
if the venue holding the event didnt serve alcohol how much would that effect attendance? i think this would be a factor in the crowds rowdy response to the fights but would it be too far?