"But that does not relieve each individual Christian of the responsibility to dig deeper into God’s Word, for the purpose of getting the full depth of the thoughts explained. This involves looking up the scriptures cited. It means reading the footnotes in Watchtower articles, some of which refer the reader to an older publication that provides a fuller explanation of a certain passage or prophecy. It requires digging deeper, putting forth effort to locate that older publication and then studying the pages referred to. It consists of making full use of specialized Bible study aids that the "slave" class has made available over the years, such as indexes, concordances, Aid to Bible Understanding, and "All Scripture Is Inspired of God and Beneficial." Yes, Christians have been provided with excellent digging tools, the latest of which is the new English-language Reference Bible, which, in time, will be available in a number of other languages. Let us use these digging tools to good advantage."
Notice how the tools they refer to are only Watchtower publications. If someone truly wants to dig for the truth, they will look at everything available on the subject from all sources. But by the Watchtower trying to limit them to only their publications, they are trying to control what they believe. Most witnesses are not going to pick up on this and would even say the Society encourages them to make sure it is the truth.