Why do Mormons insist that you only get information on their church from Mormon approved sources?

Because they want to censor what you learn, saying If you want to know about a Mormon ask a Mormon is like saying if you want to know about pedophilia ask a pedophile. The Mormon cult is about control, censorship and compliance. A good piece of advice is to get both sides of the story if you want to know about Mormons, ask both a Mormon and a former Mormon.