It’s for camaraderie, morale for the crowds. It’s sort of a way of saying, “we’re all one, let’s have a good game, we’re thankful for being able to play this sport, without our military this might not be possible so were grateful for them too, and oh were damn proud that we have this nation, it’s not perfect by any means, but we must acknowledge that with out the horrible past their is no beautiful future sort of thing”
It’s funny how people call us the most “nationalist” nation in the West. Us being nationalist compared to China, N. Korea is a complete joke. Furthermore U.S. sports fans tend to be more respectful than most when it comes to nation vs nation crowds. Not even with NFL fans are they as crazy as soccer fans in Brazil, Euro, Africa. (You guys riot onto the field and kill each other haha, it’s barbarism)