Dear god, its DUNKIN Donuts, not Duncan Donuts. As a life-long Northeasterner that killed me.
Joking aside, you are right that Dunkin Donuts is on just about every corner, but only in the Northeast. My small hometown of about 23,000 had 4 of them in the city limits. Try going south of Virginia or west toward Ohio and they are nonexistent. Its really only a Northeastern thing, although that is changing. If you go down South, its all Krispy Kreams, which is nasty, but I may be biased.
As for the American flag thing, it is true that we display our national pride more than most others. When my friend came to visit from Argentina he was surprised how almost every business and most houses had American flags flying from them. It doesn't have to do with a superiority complex, it has to do with pride.