>I do have to agree with the site that they might not be able technically to give the info.

I have mixed feelings about sites that rely upon the "we regularly purge identifying information" excuse. In this case I believe it makes the site directly responsible for libel as they are actively looking to protect the sources of information that benefits them.

I don't see how that can be if they're doing it with all comments.

They do it for all comments to have a protection when someone comes gunning for some comments.

This isn't about someone defending a champion of free speech or a hero of mankind. It's about, in most cases, liars who invent claims in comments to support their side. Such lies gain truth through repeated assertion (remember how RIM couldn't believe that the iPhone was possible? Retold endlessly, it was all based upon a quickly deleted claim on some random message board by some anonymous person claiming to be a RIM employee. The claim was absurd -- the iPhone used the same stock ingredients that RIM was used to -- but there you had an "insider" to support it).

People are bad about rumors, but there's still no reason to try to block anonymous commenting. If someone wants to make themselves anonymous for a single post, they will, barring blatant mistakes. We should not expect server logs to let us snoop around a comment made with exactly zero reputation or trustworthiness.