When the U.S. plays some Central American team on home soil and sees itself outnumbered 5:1 in supporters in the stands, it's not because all those people are Honduran or Guatemalan citizens in this country illegally. Even if Arizona wants you to believe that.
Those national identities, even if past or through ancestry or whatnot, give people clique identities within broader contexts. Look at Spain, for example. Even with Euro 2008, you could still make the argument that being Catalan or Basque is far more important than some Spanish identity.
I think the piece that Andy might be missing is that it's human nature to want to belong to some subset or clique. America is too broad of a melting pot for many to always identify with so closely. Hence why the terms Mexican-American, Italian-American, Ukrainian-American, etc., etc. Whether it's a street gang in West Side Story, North Siders vs. South Siders in Chicago, or a U.S. citizen cheering for Honduras when they're playing USA in Columbus, OH, there's a human psychological need to identify with a niche social affinity.
Pretending that doesn't exist, even as illogical as it may seem, is a bit of a naive way to look at the world. Supporting a club or an NT always involves some form of personal identity. And those definitions of identity don't always start and end with a passport.