When a doctor diagnoses a patient, who makes the decision about treatment? Perhaps the patient defers to the knowledge and authority of the doctor: "you're the doctor, you decide." Although the structure of power dynamics between the doctor and patient can bias decision-making, the final decision usually comes down to money, which is decided by the insurance company. So how does a large insurance company decide what procedure to pay for?
Others have pointed out that this is a system of misaligned incentives can lead to situations where, for example, insurance companies would be much more worried about over-prescribing rather than under-prescribing treatment. So how do insurance companies do this in practice? By conducting a scientific review of all published literature on a given treatment, and then evaluating it. For example, Aetna's Clinical Policy Bulletins lay down the law of what is, and isn't accepted medical diagnosis and treatment.