Uhh, actually, you do choose. I chose myself as someone introduced to soccer at an early age by my father, someone who didn't really care about US soccer, to cheer for the US in '94. I actually understood that yes, I am an American, not a German, even at the age of 9. And I even sparked my dad's interest in the game, eventually, through my love for it. Can you believe that? I sure can't. Most of us are dumbasses over here, but at least we know our nation.
Most of the folks around the world do not fit your description of "family ties" to a particular nation. Most of the time you see folks from the Arab world or Asia supporting sides like Italy, and I can guarantee you that their parents were not making their children say "mama mia pizzaria" at 2 years old. Perhaps some folks that have Italian parents had views pushed down their throat, so I can see why they'd like the Azzurri. That's much, much better than others, no doubt.
I usually don't have a problem with people who don't assimilate. In fact, I try to reach out to them -- yes, even Mexicans. But as a fan of US Soccer, I can't just sit here and allow this shit to happen. We need more support.