Seems like it would have to come with a fairly complicated set of rules about what sort of crowd reaction is appropriate under what circumstances, and at what volume, etc, and that would be a nightmare to "officiate". You always hear about visiting teams wanting to "quiet the crowd", but if the home team gets to control what the fake crowd noise sounds like, that would never happen. Also, I'm sure there would have to be a rule against "boo'ing" controversial penalties and other calls (and non-calls).