They have NOT come from the WTS but rather (at least originally) from ELDERS (or other members of the congregation) to whom the letters were addressed, who have then made them public, as is their right, even though instructed not to do so.
If a letter is not addressed to Elder Judas Iscariot or to Joe Pub about non confidential stuff but is addressed to a WT agent acting in his official capacity or to The Body of Elders, the letter and the information the letter contains may be considered to be wt property and it may not automatically be a person's right to make such letter public. Another word of caution, intellectual property must not have to be copyrighted to be protected.
Suppose, a wt letter is deemed to be wt property and not the property of an elder or of another person that took the letter, how does this affect a third party that received the information and published it without the permission of the owner? It depends on the substance of the letter and other variables but possession of information that is not yours does not automatically give you the right to publish such information even if the information is not copyrighted. ( wt letters may or may not be.)
A congregant may not automatically have the right to publish a letter sent to The Congregation.