Originally Posted by sortie

They do it to say "If you don't agree with me then you are anti-american".

For example : I tell the guy next to me in the bar that his shirt is ugly.
He says "bullshit, my girlfriend bought it." Then I laugh.
Then he's says "Aint nothing funny buddy", then he clinches his fist in the air and starts chanting USA, USA and the crowd starts chanting USA with him then they all tell me I suck and better leave.

That's basically how it works.