It's great to see fans jacked up like that over soccer! Why can't it be like that in America?
Did you watch the Seattle Sounders opening game? It can be, and was. The problem I've seen, is that generally you have a few home fan groups, and maybe a couple of the road team who travel to the game and help to bring other fans there. The rest of the audience is filled with the S. American crowd who is generally into it, and then the crowd of 'I don't know how to have a good time at a football game.' This latter crowd usually includes soccer moms, disinterested kids (who treat it like a baseball game), and people who just want to root for a local team but really have no genuine interest/love for the game.
Americans also generally know jack all about football. Without good TV exposure (if ESPN ends up showing Premier league games on the possibly ex-ESPN Classic [possibly changed to ESPN 3] this could happen) people will hopefully learn enough to appreciate what is going on. To appreciate good plays in the midfield, and quality scoring chances.