If you think it's offensive for a Muslim group to exploit the 9/11 atrocity, you're an anti-Muslim bigot and un-American to boot. It is a claim so bizarre, so twisted, so utterly at odds with common sense that it's hard to believe anyone would assert it except as some sort of dark joke. Yet for the past few weeks, it has been put forward, apparently in all seriousness, by those who fancy themselves America's best and brightest, from the mayor of New York all the way down to Peter Beinart.

What accounts for this madness? Charles...