We don't give "far more" than Japan; for 2004, we are scheduled to give $363 million, and Japan is scheduled to give $280 million.
Seeing as our population is well more than double Japan's, we actually give less per capita than Japan does:
As to why we give any money to the U.N., it certainly isn't because it is a good idea or because we get good value for our money.
The U.N. is proving to be a malignant version of the old League of Nations, which was just as impotent, but not so uncivilized as to cheer baldly murderous dictators like Robert Mugabe.
Perhaps President Carter or President Clinton feels comfortable in the U.N.'s halls, but President Bush did exactly the right action, and not a criminal one as Secretary General Annan recently suggested, when we took unsanctioned multilateral action against the evil dictator Hussein. Had we waited on that knitting society to take action, Hussein would still be in a palace, not in a jail cell.