There is only one Vocaloid that has connections with America (United States), the Vocaloid3 Oliver, and even then he is a joint effort between Utah-based company VocaTone and Swedish company PowerFX. Miriam Stockley who offered her voice to Miriam is from South Africa. (Later, she moved to England, then to America). With the origins in place the voicebanks (along with the avatar) appear to be designated to it. Also it is a common reaction for people, in general, to associate "English" and "Western" with America(ns).
For a while it was believed Big Al had a Swedish accent since he came from a Swedish studio. However, many questioned since because of the accent sounded American; Big Al has been confirmed to carry an American accent, Frank Sanderson is American, but he spent much of his life overseas. Sweet Ann's voice provider was identified as Australian, however, it is now indeterminable if she has the accent of her providers country of origins.