We have american themed restaurants here (there is one in my town and another on the road to the next town)...they are both pseudo 50's themed, and the main draw is the variety of burgers and milkshakes, but otherwise offer the standard sort of stuff (fries, cokes, hotdogs and rock n' roll on loop). I don't think many of them are chain restaraunts, from the ones i've seen they're mostly local/family type passions rum by folks who really like "happy days" and "grease". Although frankie and bennies' is a chain, it is a kind of "interwar-post ww2 new york italian american" theme so quite restricted in cultural/regional flavour. There are some more i think are american chain restaraunts, but they are generic enough i wouldn't describe them as american themed.