I believe that Matt's answer is correct. While I do not recall the precise citation to the law, as I recall, sometime around 1998-1999, Congress expressly exempted ISP and content providers from any civil liability for content of third parties. I looked closely at it once upon a time and it seemed very broad and inclusive. I have not looked at it in years.
As for deletion of the messages, I also think anyone would be hard pressed to make a claim for damages against the provider of posting services. First, on most services, and Bob or Matt can correct me if this is wrong, deletions simply remove the posts from public view rather than truly deleting them. Thus, they remain available if they are needed in the future for litigation of any kind. Second, death threats (even veiled ones) will almost certainly violate all sites Terms of Use and thus the service is within its contractual rights in deleting them -- regardless of whether it is requested by anyone.
An argument could be made -- and I have no idea how legally successful it could be -- that once a provider has actual notice of conduct that is threatening or otherwise illegal, such as by deleting posts based on content, it may have some exposure by continuing to let the threatening or violating party post. I have not thought this through fully, but even if they had some kind of theoretical exposure, it would be difficult for the person threatened to prove damages caused by the provider. It would seem that the threatened party would be better off knowing about the threatener's intentions than being ignorant of them. At least in part, this potential exposure is why most providers will not delete posts based merely on someone's assertion that the content was untrue. Service providers are not in the business of regulating content, except when it violates their contractual terms of use.
One final comment: these are my personal (not professional) views and should not be taken as legal advice. No one should rely, act, or not act based on anything I have said. These views are worth exactly what has been paid for them. I have not viewed the specific example you asked about and would not offer anyone specific legal advice without doing some serious research -- for which I would require and expect to be paid.
Troy
Those who shoot from the hip usually end up just shooting themselves.
Plan the grub and grub the plan.
Where is the party tonight? Who is bringng the drinks?
