DoJ's Gag Order on Reason Has Been Lifted
This happens all the time.  At reddit we would get requests from law enforcement asking for email addresses and other private information.  Luckily in our case we could simply reply that we didn't know, since we didn't require any personal information to sign up and didn't keep IP address logs that long.But having been on the other side of the coin, investigating computer crime, I can tell you why it happens.  It's really easy to make a request for information, and in most cases, the person you're asking will just willingly give it up even though they don't legally have to, either because they want to be helpful or because they don't know about their legal rights.  Even if the evidence can't be used to build a case against the person in court, it can still be used to lead down a path towards finding the person and gathering evidence that is admissible.