There's nothing big about that. The US is a nation of immigrants by birth or descent. If one wants to see a packed house for sports, just schedule an Ireland vs. Italy match in the NYC metro area. One is scheduled again for a soccer match in July. I regularly see people wearing Irish or Italian sports gear by Americans of such descent in that sport or another. It has nothing to do with disloyalty or routing against the US in sports, just one additionally supporting another team too given their heritage.