The answer is obvious -- Ellison is a Muslim. And whoever decides these matters, not to mention virtually every editorial page in America, is not going to offend a Muslim. In fact, many of these people argue it will be a good thing because Muslims around the world will see what an open society America is and how much Americans honor Muslims and the Koran.
This argument appeals to all those who believe that one of the greatest goals of America is to be loved by the world, and especially by Muslims because then fewer Muslims will hate us (and therefore fewer will bomb us).
But these naive people do not appreciate that America will not change the attitude of a single American-hating Muslim by allowing Ellison to substitute the Koran for the Bible. In fact, the opposite is more likely: Ellison's doing so will embolden Islamic extremists and make new ones, as Islamists, rightly or wrongly, see the first sign of the realization of their greatest goal -- the Islamicization of America.
When all elected officials take their oaths of office with their hands on the very same book, they all affirm that some unifying value system underlies American civilization. If Keith Ellison is allowed to change that, he will be doing more damage to the unity of America and to the value system that has formed this country than the terrorists of 9-11. It is hard to believe that this is the legacy most Muslim Americans want to bequeath to America. But if it is, it is not only Europe that is in trouble.