It's probably a cultural thing. From my vantage point outside of America, chanting support for your country like a sports team feels awkward and myopic. It's not something most other civilised countries do — I associate it more readily with failed/fascist/dictatorial regimes.

I think it is a cultural thing, but thinking about it now it would sound really similar to how you describe if they were chanting "America" instead of "USA".

I don't know, but the point is that I can see where you are coming from. I think that the "USA" thing has become "normalized" to me because of other things in the media (sports, wartime, etc...) that it doesn't phase me, but changing the wording makes it pretty clear.

If you're interested in reading/thinking more about tribalism and people reacting to displays like this, I found this essay really interesting: http://slatestarcodex.com/2014/05/19/nerds-can-be-bees-too/