See, I've thought the opposite about the focus of the game on the Americas. The Guerrilla team is definitely not American, they are based in Europe. I thought the focus of the game geographically was more culturally driven. Europe has been devastated by two World Wars, which the effects of are still seen and sometimes experienced today. I thought they didn't want to market a game showing a yet again (and worse!) devastated Europe, so the focus on North America was both to provide a geographically huge area they could utilize as well as putting the devastation and decay on North America for a change, which has never really been touched by those terrible world events in the past.
I had actually assumed that the first game was situated in mid or Eastern Europe just from some of the flavors of things, until you find the football stadium. Then the location being North America was pretty obvious. Also, from the game story, it's North America that creates the force that ends the human race, so....not exactly a "Go G.I. Joe!" type of plot. The themes of technical innovation killing, both on purpose and unintentionally, the power madness that drives the invention...all of that. Actually pretty damning on "American culture" if you ask me. But then again, I live here and don't have as much exposure to anti-American sentiment as a lot of people in other countries do. I still saw that message though. But you know what? Despite it all....it's just a game. Just entertainment. Sure there can be some morals and story to take to heart, but no one should really put a lot of stock in things that come from these games.