Who's "they". As I mentioned before, from the folks I know who work in US TV, it's all about scheduling and season lengths and nothing at all to do with xenophobia, despite what random theories folks like to speculate with on the intrawebs. People on the whole are surprisingly not stupid, and handle things not in their own exact accent just fine, with minor exceptions.
Broadchurch is on BBC America? It's not a BBC show; it was made by ITV. Weird. I don't really know anything about BBC America, though (not a request for information).