Are the elders letters copyrighted?
As to elder's letters (or letters to the congregation), here's my view, for what it's worth: they are not copyrighted. How can you copyright a letter?
EG. I write a letter to @Sparrowdown including some very personal details, maybe even a photo. If he then goes and shows that to all and sundry, then I have no leg to stand on, as that is HIS prerogative. The letter is addressed to him so therefore becomes HIS property, not mine. What he does with it afterwards is entirely up to him, even if that entails embarrassing me as the sender (that should teach me to think more carefully before sending anything that could incriminate me, in a practical sense.)
Where have the letters come from that are available online from places like www.avoidjw.org ? 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.
As to videos and publications, then that's a whole different kettle of fish as you are dealing with copyrighted intellectual (I use the word very loosely!) property.