It's their divine right, handed down in a direct lineage from Paul to Pastor Russell to the present day Governing Body. The word 'Watchtower' has been lost over the years through mis-translations, along with Blessed are the cheesemakers and other such insights. Don't worry one day the watchtower will make the ancient scriptures obsolete and we'll need rely only on their literature for spritual inspiration.
I hope this answers your question, welcome to the board.