First, because liberals judge social policies largely on the basis of how they feel rather than how policies actually play out in real life. It felt good to show how committed liberal universities are to freedom of speech and it felt good (for some on the left) to give Ahmadinejad hell.
Second, because today's liberals are, by definition, naive about evil. Since the Vietnam War, the American left, including liberals, have been naive about evil, just as the European left became naive about evil after World War I.
That is why there was universal liberal editorial condemnation of President Ronald Reagan for calling the Soviet Union an "evil empire." That is why there was universal liberal editorial condemnation of Israel for destroying Saddam Hussein's Osirak nuclear facility. That is why prominent liberals -- with no exception of which I am aware -- all condemned President George W. Bush's use of the term "Axis of Evil" to characterize the regimes of Iran, North Korea and Saddam Hussein's Iraq.
In a nutshell, Columbia invited a Holocaust-denying, homosexual-executing, women-suppressing, genocide-advocating, terrorist-supporting national leader because its president is the quintessential liberal. He is a man with many good intentions who, like other liberals, judges policies primarily by intentions, and liberals know that their intentions are noble (that's why they rarely acknowledge that conservatives can be good people).
But intentions matter little in policy making. Wisdom matters infinitely more. And there is little wisdom on the left.
There is little wisdom not only regarding evil, but regarding taxation, the size of government, illegal immigration, the effects of affirmative action on blacks, bilingual education, male-female differences, boys' needs, high school textbooks (revised in the name of multiculturalism), reasons for violent crime and terror (unemployment and poverty rather than awful values), the promotion of self-esteem in schools, early sex education, early withdrawal from Iraq, and just about every other major social issue.
In each case, just as in the disastrous invitation to Ahmadinejad, liberals feel good about their intentions and therefore about their decisions. But few, if any, of those decisions are wise. This is not surprising. A generation whose primary goals have included overthrowing Judeo-Christian values, which once said, "Don't trust anyone over 30," and which has rejected external moral authority (God, parents, teachers, religion) is not going to be wise. And absence of wisdom is why Columbia University and the New York Times thought inviting Mahmoud Ahmadinejad was a good idea.