Wired has an excellent article on why harassment policies are needed — it’s because the social dynamics of conventions can mess up people’s perspectives.
Conventions need anti-harassment policies. Not because convention attendees are disproportionately boorish or creepy–they’re really not–or because of social obliviousness. Rather, the difficulty lies in the very thing that makes conventions conventions: the social phenomena that come into play whenever humans gather in large groups.
Read the whole thing. It’s very thorough in discussing the psychology behind harassing behavior, and why it’s not just something evil psychopathic trolls do.