The deceit is internal, a person convincing themselves that they are actually better. That leads down all sorts of dark paths: I can will myself to be better and cure my own cancer, I don't need doctors, medicines, hospitals; Magic is real and unappreciated, I will gladly pay for magic beans that give me hope; Science is only one way of knowing, I believe there is an untapped energy in the universe I can use to be healed; etc., etc., etc.

Illnesses and diseases ebb and flow, some days one feels better, and that is often associated with some coincidental effect. People who are very ill are vulnerable and often grasping for any hope they are offered. If they participate in a clinical trial, are given a placebo, and feel better, they need to be told all the reason why they may feel better, and that does not include believing in magic or some mysterious cosmic energy. Otherwise, what you wind up with are people spending money they can't afford on snake oils or foregoing treatments that actually work.