I think this is completely relevant. I think a company should be able to moderate it's own content based on what it deems acceptable for it's community. This is a totally valid question and something I believe is worth discussing as we have gone well beyond absurdity at this point.
Edit: Disclosure, I am a twitter shareholder and approve the ban of @realDonaldTrump from twitter unless the "libel" and McCarthy type posts are removed and any accusations as such are filed in proper mediums.
There is something to be said though about shining a spotlight on the cockroach infestation, rather than filling the holes with spackle followed by a nice paint job.
Threats of violence, or calls that encourage it are a fair line to draw for any user. But the posts you're talking about removing, I think amounts to turd polishing that might actually make it sparkle more than it deserves, and ends up being a deception of the true character of the person.
Imagine if all the birtherism posts were scrubbed? That's his true nature. Compulsive liar or delusional - doesn't really matter. My concern is assurance posts are difficult to scrub. That's what incentivizes people to be sincere rather than just taking inconsequential pot shots at others.
Edit: Disclosure, I am a twitter shareholder and approve the ban of @realDonaldTrump from twitter unless the "libel" and McCarthy type posts are removed and any accusations as such are filed in proper mediums.