In any circumsatance, they can not copyright the using of "Watchtower" (like in "Watchtower Observer" or "Quotes-Watchower") for the term "Watchtower in not for themselves. That's what they themselves wrote in the Proclaim book (WTS 1993, page 48, footnote):
"The expression "Watch Tower" is not unique to Russelll's writings or to Jehovah's Witnesses. George Storrs published a book in 1850's called The Watch Tower: Or, Man in Death; and the Hope for a Future Life. The name was also incorporated in the title of various religious periodicals. It stems from the idea of keeping on the watch for the outworking of God's purposes._Isa.21:8,11,12;Ezek. 3:17;Hab. 2:1." (quote from "Proclaim book, page 48, footnote)
And a couple of Mormons and Adventists "periodicals" had the same name
In addition, if you use google serch engine and type "Watchtower," you will find out heaps of companies take this "Watchtower" name. Do they sue all of those companies ?