It wasn't on private property -- it was on the campus of the university in Florida, of which he is a member, in a public event with a microphone for comments.
He was not asked to leave by the owner, and arguably could not have been anyway, given that he was a student expressing a legitimate opinion in a campus event that was promoted as an open forum.
The Libertarian National Committee's new communications director was part of a similar debate at the campus of his alma mater in North Carolina. In his case, his group was arguing for the right to advocate against same-sex marriage.
While I obviously (vehemently) disagree with the position of his group on marriage, I equally vehemently disagreed with the position of the university in censoring his group's activities. I believe that said university is public, as well, but even if it isn't, students have a right to express themselves on university facilities if they're paying tuition (that's part and parcel of the consideration received for tuition). I applaud his group for calling attention to the double-standards of the university and successfully challenging the university's policy.
On the actual issues themselves, be they gay marriage or 9/11 or the 2004 elections or what have you, the victor should be determined by the free market of ideas, not quasi-public "police" who report to a quasi-public "institution" that decides that it is a private venue for one purpose but a public facility for another.
And from a moral perspective, neither Giuliani nor Kerry should be willing to use (or promote the use) of police to censor or intimidate people who ask them tough questions, no matter how "obnoxious" or "crazy" they appear. After all, the next person who gets tasered might be a "crazy, obnoxious, out of the mainstream" LP member asking tough questions about Iraq or the gold standard.
Cheers,
Brian