I disagree. U.S. cheers for many Brazilians over Americans. The Chael/Silva line was close to a split in my estimation. They love Big Nog and Wandy. Love Aldo and love JDS. I bet JDS has more fans in America than Cain. U.S. likes fighters who go out and give it their all trying to put on a show for them. They may not be the most educated MMA fans, nor the best. But bias they aren't.
You will hear USA USA every so often. But maybe for a guy like Chuck, Hendo, or Randy. You would never see them do that for Jake Shields (haven't fact checked that).
Basically if you are a guy who is a diva or a guy who isn't "tough" you won't get nearly as much love from US no matter where you come from.
Take this forum for example. U.S. surely has the highest representation, but yourself and other Canadians are basically GSP fans of some sort. English posters at least have some love for Bisping if not out right love. Aussi's love their own as well here. And you can seemingly identify who is Asian as they love all who are Asian. But myself for example, I love Wandy and Anderson. Love Mark Hunt. Those are some of my favorite fighters. I routinely root for guys like Palhares and Gunnar Nelson.
American fans are the least bias in my opinion. Hands down. It could just be a numbers game, with so many American fighters and venues. But you rarely see the UFC needing to stack American fighters on American cards to sell. People here will get just excited for Big Nog as they are for Mir. Most likely more so.
I don't know man, it's hard to tell who are the most patriotic and who the least as it can be expressed in different ways, but it's obvious that US-Americans are pretty patriotic. There are not many other nations that claim to live in "God's own country". And most other countries don't have a Pledge of Allegiance routine in school. I also can't remember to have ever heard the equivalent to the "U-S-A! U-S-A!" chants when a UFC card took place in another country. No "CA-NA-DA! CA-NA-DA!", "U-K! U-K!", "DEUTSCH-LAND! DEUTSCH-LAND!" or "SVERi-GE! SVERi-GE!" chants in the respective countries. You can't make it more obvious when you don't just cheer for a fighter of your own nationality and shout his name, but instead shout the name (or its initials) of the country that you disconnect your support from the fighter's individual personality and support him merely, because he is a fellow countryman.
It's true, in this forum it's a bit different and US posters also support non-US fighters, but keep in mind that forum members here are rather educated about the sport and the fighters, so it's more likely to find a supporting connection on the fighters' personalities beyond their nationalities. That's different to the mainstream fan. (And it's a bit mean to point out that CP is a fan of St. Pierre as St. Pierre is a fighter who makes it pretty easy to become a fan of, regardless of his nationality. Also you shouldn't drop the fact that CP is a pretty strong supporter of US' own Chael Sonnen
And the UFC doesn't need to stack US cards with US fighters, because usually the majority of the fighters are US fighters anyways. And there is less need to expose US-fighters to their homecrowd when there is a show every fortnight and not only once or twice a year. You can see the drop of interest in boxing since the important HW titles are held by guys from former Soviet states for a while now.