Is whats going on in that video technically allowed? I work at a tourist attraction and if that were happening outside we would more than likely split it up. If its a large group and they're loud, simply because its a large group and they can't help it thats fine, but in this case the guides were instructing them to sing/dance. I know Disney obviously doesn't do anything about it but I would assume they have some kind of policy that technically that should not happen. If that were to happen outside of where I work, we would break it up, and explain that it is our private property, if they want to put on some kind of street performance or have a group program we'd be happy to work something out in advance, but its not okay to just show up with 50 people and have them do some kind of routine.
Even if a large group is quiet or well behaved, a lot of people are still intimidated just by the sheer number of them. A lot of places, like malls, museums, etc... would break up large groups into smaller groups, I guess it would be un-magical for Disney to split people up, but there must be some kind of official policy regarding the tour groups.