When the forerunner of the Insight volumes (Aid to Bible Understanding) was written back in the 70's by Ray Franz and his friends, studying the bible on it's own really took off in Bethel, resulting in many disfellowshipings. It seems that the GB/FDS don't actually approve of reading it on it's own, despite the fact that the bible itself clearly states we don't need anything or anyone else to teach us except Jesus Christ through the Holy Spirit.
The whole reason why they are so scared of us doing it is that they realise once we've read the scriptures with an open mind we reach the conclusion that they are teaching us a load of falsehoods that go directly against what the bible is telling us. Once we've discovered that then they lose their obedient slaves.