While private insurance companies have to negotiate individually for the rates they pay for procedures within their networks, Medicare handles that function by a Congressional mandate as to what prices will be paid to providers (presumably along with a prohibition on doctors refusing to work for payment which doesn't cover their costs of performing some services). When Medicare rates force providers to operate at a loss, the difference is made up by jacking up rates to private insurance some, and to the uninsured even more. The U.S. medical care system effectively subsidises proce controls in other countries through a similar chain reaction.