Amazon CEO Jeff Bezos wants to be the arbiter of acceptable thought. At least, that’s the warning cry after Amazon’s recent decision to ban the sale of several controversial books about conversion therapy.

This includes books by Dr. Joseph Nicolosi, a practitioner of conversion therapy and famous figure in the “ex-gay” movement. His titles, including works such as Healing Homosexuality and Reparative Therapy of Male Homosexuality, are still available through their publisher. Amazon’s decision also affects several books by people who say they have recovered from homosexuality and no longer consider themselves gay.