Clearly Humbert has been obsessed with America from an early age, exposed as he was to idealized images of its grand panoramas and natural wonders. His trips across America in Lolita detail this fascination (and repulsion). He scrutinizes every absurd tourist trap, crappy motel, consumer habit, national compulsion, and stereotype of American culture. He both rejects the fussy, musty ways of Europe and plays upon American perceptions of the sophisticated European intellectual. To him, Charlotte represents the worst of American culture: an unthinking, mediocre, upstart with pretentions to cultural sophistication. Still, he is all too ready to let her fantasize about his European background.
Though Humbert recognizes all of Lolita's bad-mannered, outspoken, brash American-ness, he gives her a pass, and in fact embraces her slang and love of lowbrow magazines and Hollywood movies. Unlike her mother, she utterly repudiates, even mocks, his pretense to cleverness and refuses to be reformed or refined by him. Humbert often covertly associates Lolita with America, praising their shared youth and vulgarity. His admission that he has defiled America bears a strong parallel to his treatment of Lolita. Nabokov himself was hurt by accusations that the book was "un-American," an assessment, in his words, that "pains me considerably more than the idiotic accusation of immorality" (source: James Kincaid, "Lolita at Middle Age"). America is the land of mass culture, a modern society of consumer goods, a nation of tourist sights and souvenirs, where everything is commodified and collectable.
Even as Humbert mocks American consumerism as a cultural habit, he engages in a sinister form of it by using up Lolita's childhood.
The American setting is absolutely crucial to the novel's themes of youth and consumerism.