It's surprising to me that everyone says "go the other way". Or rather shocking.... Why doesn't Disney address this?!
$$$$$$$ Or Economics as a PP stated. These BTGs represent a big chunk of cash to Disney. They don't want to risk losing their business by imposing restrictions on their movements or actions. Although the likelihood of them actually having the BTGs go elsewhere if reasonable rules were imposed is highly debatable. It probably has something to do with imposing rules on one type of group without imposing them on all guests????? Pure conjecture on my part.
As for the chaperones?

They aren't. They're paid (and probably poorly paid) young tour guides paid by the tour company. It's their job to keep these kids happy, not to monitor their behavior. Unfortunately. And obviously the tour companies are only interested in getting paid, and they wont' have customers if they get poor reviews from previous customers. And they won't get good reviews from the kids if the tour guides try to restrain the behaviour of these kids.
The end result is a large group of teenagers without any parental supervision allowed to pretty much do what they want. Gang mentality takes over. It's no different than what would happen with any similar group in a similar situation. It just happens that this particular group has become so dominant in recent years.
Lots of folks have recommended complaining to Disney and asking that they be 'restrained' (either their group size or their behavior). Look at it from Disney's side:
Multiple BIG groups of paying customers who return yearly versus a small number of small family groups who may or may not return again at some future time. Who wins? Not me, that's for sure. We don't travel to WDW in July because of the heat. We will try to avoid January in the future ONLY because of how unpleasant it is to be surrounded by these BTGs.
One last thing- I know that we've been specifically talking about Brazillian TGs, but I have no hesitation in saying that I think it just as easily translates into any BIG Tour Group- no matter what the nationality.