On the letter, Roy Black plainly states in bold type that "this letter is being sent as part of settlement discussions and thereby confidential pursuant to FRCP 3.172(h) and F.S. 90.408" (see http://www.thesmokinggun.com/graphics/art3/rushletters1.gif)
By following the links above and those provided by Cubs Fan, you will see that both references describe the inadmissibility of plea negotiations as an admission of guilt. For example, Florida Statutes 90.408:
90.408 Compromise and offers to compromise.—Evidence of an offer to compromise a claim which was disputed as to validity or amount, as well as any relevant conduct or statements made in negotiations concerning a compromise, is inadmissible to prove liability or absence of liability for the claim or its value.and Florida Rules of Criminal Procedure (2004) FRCP 3.172(h)
(h) Evidence. Except as otherwise provided in this rule, evidence of an offer or a plea of guilty or nolo contendere, later withdrawn, or of statements made in connection therewith, is not admissible in any civil or criminal proceeding against the person who made the plea or offer.So I take it [I am not a lawyer, I didn't even sleep at a Holiday Inn last night] Atty. Black's position is that since the evidence is inadmissible, it is therefore confidential. I suppose there is case law to back that up. However, even if that isn't true, the fact that the State Attorney's Office (SAO) didn't bring that up in discussions with the Bar Ass'n. and the AG's Office seems like a major omission.
So what is the SAO's position on this, "We're not mean, we're just stupid" maybe?
