Universities are public forums for discussion where ideas can be freely exchanged, just like sidewalks, street corners, and parks; you can check with the ACLU. That said, controversial speakers holding very unpopular views are protected under the First Ammendment to speak their mind on campuses, given that they don't make threats to other people, slander, or incite violence. People such as Holocaust deniers, creation scientists, Ayaan Hirsi Ali, Bill Mahr, Condoleeza Rice, Milo Yiannopoulous, Ann Coulter, the Westboro Baptist Church, and campus preachers have the right to speak on a campus just as much as we have the right ignore them. After all, nobody is being forced to attend. As a paleontologist, creation scientists are free to speak on my Alma Mater, but that doesn't mean I have to attend; I can ignore or peacefully protest their beliefs without violently stirring a riot. And most importantly, universities are not safe spaces.