Good question. I was looking for a carrier and device combo that had HSPA+ speeds or better. At that time I had checked BVI and Antigua but hadn't done any research on SXM.

On another note I found this gsm world coverage map - it shows how North America and parts of South America are on one set of frequencies and the rest of the world is on another set. From my reading it wasn't that the US wanted to be different, there were practical reasons why those frequencies were use (like the bandwidth was already in use). I am not saying that a quad band device wouldn't work in most cases. GSM World Coverage Map