The court … in this case … did not decide that posting a cease-and-desist letter is copyright infringement (which would have required considering the fair use defense). Rather, the court was only asked to decide whether the plaintiff could use a subpoena (under 17 U.S.C. § 512(h)) to discover the identity of the poster. The court concluded that for this, the potential plaintiff only had to show that copyright law presumptively protected his work (which it does); the fair use inquiry would then take place when the merits of the case are litigated, at trial or on a pretrial motion.
If it weren’t for the unpublished nature of the letter, the Supreme Court’s Campbell v. Acuff-Rose decision, on which I rely in my quick analysis above, would make this an almost open-and-shut fair use case. The unpublished nature of the work undermines that in some measure (see, e.g., Harper & Row v. Nation Enterprises); but I still think the copier’s fair use case is quite strong.
The rest of the article is worth reading: Copyright Law and Cease-and-Desist Letters.
Update: a court ruled against the fair use defense.