This topic was covered non-stop many years ago on CompuServes Florida forum. Although I never ran across the "Brazilizn Tees ( T-Shirts that is, with their flags)", the problem as I understand it is minimal to no adult supervision.
Imagine a large group of American teens on vacation in Rio with little to no adult supervision. I would think a good portion of them would be perceived by the locals as unruly, rude, obnoxious, etc...
While the kids may be rude I lay the blame on the adults / parents who allow the kids to roam WDW unattended.
Just my $.02
steve