Technically, groups can stay anywhere they want, provided they're willing to spend the money to do so. It's just that most groups tend to make the trip as affordable as possible to everyone in their group, which means Value resorts or off-site. I suspect (though can't confirm) that when tour groups arrange trips with Disney, they tend to just book the trip at All Stars, or at least suggest it. Besides, for tour groups pinching pennies, All-Stars is a teensy bit cheaper than Pop due to the change in resort tax 'twixt the two counties.
But a Brazilian tour group could opt to stay at Grand Floridian if they wanted, as long as the rooms were available and they could pay for it.
No one listens to me, probably because of my criminal record, but if I ever had the ear of a WDW exec, I'd suggest either creating or making one of the All-Star resorts a resort geared specifically (though not exclusively) for large budget-conscious tour groups, the same way Coronado is geared specifically (though not exclusively) for convention-goers. Of course, families who wanted to stay at that resort shouldn't and can't be excluded. But if, when booking their stay, those people were advised that it's predominately for larger chaperoned tour groups, I'd think most people would have the sense to say "oh, ok, what else ya got?"