I always took the accents as an example of the British perception of Americans appearing "cultured" by copying their lofty Britishisms. An example would be the myriads of modern Americans who seem to relish using "bloody," "bloke," "randy," "shite," etc as a means to differentiate themselves as sophisticated and urbane.

Keep in mind that most of his American characters represent moneyed individuals from a land of plenty. He was caricaturizing the Gilded Age nouveau riche.

You should check out this essay.