Patients are usually ignorant of medicine.
Prognoses are often less than certain.
Placebos, which involve deception, are among the most powerful agents known to medicine. If the deception is intended to benefit the patient, it is morally justified. They point out that physicians are not justified in engaging in just any form of action to benefit patients. E.g., they are not justified in removing the organs of a healthy patient to save 4 others. It is wrong because it doesn't treat a human being with respect.