The real reason they won't respond to unsigned letters is because they want to know who it is that is questioning their teachings. If someone sends a letter and asks a question such as: "Why does the WTS say Babylon was destroyed in 607 BCE when all other sources say 587/586 BCE?" they want to know who asked that question so they can notify that person's elder's so they can get ready to disfellowship them. It's just another control tactic.
zugz