Speaking as a relatively new fan of the series, I don't see what the big deal is. Ubisoft has shown that, in both this series and Prince of Persia, they're willing to put the time and effort into making a good story and immersive gameplay that most game developers put into making Space Marine Adventure VI. Wherever they decide to put the full sequel, isn't it reasonable to assume they'll do a good job of it? I mean, we're all in agreement that the previous installments have had solid, innovative concepts and realizations thereof, I hardly think the writers of this series, who have put a lot of work into this plot so far (despite it making about as much sense as a David Lynch film narrated by Charles Manson), are going to piss it all away just to pander to the American market. American audiences, and indeed audiences worldwide, have embraced the far-flung settings the game has come up with so far, it would be ridiculous to assume they would all stop doing so now.
Also, even if I am an American, I still feel that I have the right to be insulted that people of other nationalities would profess disdain for a new title in a series that they claim to be fans of just because it makes a tour of American Civil War times. Even if I personally believe they just won't take it there, rejecting the promise a title set in it could have because (and only because) it's 'another' game in America is just as groundless and wrong as an American passing up Assassin's Creed 1, The Sabetour or any other game that's outside their comfort zone. And I'm not singling out, both of these groups of people exist and need to be dutifully ignored while the rest of us grown-ups get our game on.