I'm not a lawyer but, honestly, I don't see why Twitter is -- I guess -- fearing law-suites on this front. The terms of service and and always have been very clear on cases like this:
4. You must not abuse, harass, threaten, impersonate or intimidate other Twitter users.
Violation of any of these agreements will result in the termination of your Twitter.com account.
Basic Terms (snapshot from 2007)
3. We reserve the right to refuse service to anyone for any reason at any time.
General Conditions (snapshot from 2007)
At least to me this should be enough to warrant a warning for even the slightest offense and leave no questions open about a ban if necessary.
I've seen some of the offending tweets and at least some seem to come through an open relay -- basically a normal twitter account linked with an open form on the net where anyone can "anonymously" post updates.
Even in such a case I don't see why they can't remove at least the offending updates or make this feed private altogether. Also given the content of these tweets, I wouldn't see a problem with completely banning this account.
Ariel also opened a topic on getsatisfaction.com about this issue and the "official response" was (again) quite underwhelming.
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