They do that because they are a scam. If you only use their items, they can doctor their items to support their scam. They alter their Bible, and twist the meaning of even that, so they can "prove" whatever they want. Then they discredit outside sources, so you are stuck with the lies they want you to believe and no legitimate way to disprove them.
This happens with any group that wants to maintain status quo. Mainstream financial media do the same thing--and they are working to destroy alternative sources (SOPA, anyone?). The medical establishment relies on its peer-reviewed, drug company supported journals and will not allow independent (not drug company sponsored) material as valid proof against them. Schools use their "official" textbooks, and give lists of sources that students may use to support them (ignoring alternative sources that openly contradict their own, calling them rubbish). Why wouldn't the Washtowel Slaveholdery be any different?
So much for being better than the world.