Comments like that are generally made by people looking to get a reaction. It's not a good thing to do, but the overreaction the other way (aka "I'd like to apologize on behalf of all sane Americans.") is almost as retarded.
All people do by picking up the tweets is spread them around and give those people the attention they wanted in the first place. And to what end? Chances are very good the people that made those tweets don't actually have anything against Japanese people. If that's the case and they were seeking attention, they got it. If it's not the case and they really do feel that way, spreading the tweet around isn't going to make them feel remorseful or change their ways, and it will only spread their message of hate.
So it's a lose - lose situation to spread the tweets, or links to sites spreading the tweets, around.