Let me start apologizing for the lengthy post, but I wanted to bear witness.
I agree 100% with Redz' comment (TP) - it is indeed fruit of the provincialism. I think that the large distances (that avoid to host 10000 LA-fans in Boston during the NBA finals) help a lot.
I don't want to be trivial, but I want to add another reason: the abyss between Europe (and probably most of the word) and US in sport education. The sport in Europe is seen as THE OUTLET, not only for who plays, but especially for who supports. Many people (the "TRUE" fans) live the week waiting and preparing for the Sunday (clearly I am referring to the "ultra" of the big clubs). In this way, that moment assumes an importance that it should not have. Not to say, that -I know that I seem exaggerated, but I promise you I am not- some supporters (again the "REAL" ultra) don't really care about the game nearly as much as of the war of the stadium.
But I want to add my 2 cents fruit of personal experience, as I come from Europe... were accidents happen, and I have assisted a bunch of times to soccer events.
1. In US the only time I heard an F-word at a match, I've seen people turning from 5 rows below to see the utterer, while in Europe there are chants with F-words sung by all the stadium - not to say that they have a special "uh uh uh" treatment for black players; 2. In US I saw a guy taken away by police because he was screaming to the batter from the seats close to the box, while in Europe the curve "typically" throws (generally no more than coins) to the players (I've seen throwing a water tap to the famous goalkeeper Buffon- luckily it didn't hit him); 3. In US some Rockets dudes asked me -green shirt- to take a picture of them, after the Celtics winning match in Houston, while in Europe if I support the visitor team, I need to wait in the stadium for 1 hour after the end of the game that the homers leave; 4. In US families go together at the stadium, in Europe mostly adult men go to the stadium (it's too dangerous to bring kids).