>Actively spreading the harassing or inciting material is an implicit endorsement - you could stop it if you wanted.
Hosting an open forum is not the same as endorsing ideas on said forum.
In fact, I would argue strongly to the contrary-twitter has an implicit social responsibility to allow all dialogue, as it is effectively in a position to limit speech, given the size of its audience and its position as a disseminator of information.
Do you really want faceless corporations like Facebook, google, and twitter deciding what is and is not appropriate material? Especially when they effectively have more immediate power in censoring or creating narratives than our own government?