because American thinking tends to conflate the concepts of just cause and just tactics. The default American opinion is that if someone starts a fight, the other party has the right to finish it by any means necessary, which means that to many Americans, the only significant fact is that Hizbullah struck the first blow.
There might be some insight about the American psyche there, but I would say the main reason is years of well-organized pro-Israel PR campaigns carried out by the main pro-Israel think tanks in DC (and the absence of equivalent pro-Arab think tanks) that has shaped much American political and media thinking about the Arab-Israeli conflict. And on top of that you have to add the not insignificant number of well-established, fundamentally pro-Israel publications in the US such as the New Republic, New York Sun, New York Post, Commentary, the Forward, and arguably the New York Times. This media reaction is not just the result of a "just desserts" attitude but the long-standing presence of almost automatically pro-Israel publications in the American media.