Morally, if not legally. This blog post by Anil Dash, http://dashes.com/anil/2011/07/if-your-websites-full-of-assholes-its-your-fault.html is so very very right.
I've been waiting all day to make a post about this, and my own experience moderating communities and I simply don't have the time. In fact, not having the time is why I closed down a mailing list I ran around about 10 years ago - the nature of the debates we intended to have, and did have, quite successfully, required close attention, and I couldn't give it with Silicon Valley sucking my life away - and handed over moderation of another list for Webgrrls-LA around the same time, and never enabled comments on this blog, either. Because I didn't have the time. Don't have the time? Well then, don't host the community. Because you do have a moral obligation to police assholes, call them on their racism, classism, sexism, or general jerkiness. We have that obligation because we're human beings and we live in the same communities, even when they're online ones.