Insurance (including Medicare and Medicaid) can and do push back against things they identify as inappropriate treatment. There are many problems with this system -- too restrictive in some places, not restrictive enough in others -- but in theory this could be improved to act as a reasonable check on rogue doctors.
Another relevant effect worth noting here is CYA medicine: doctors order tests aimed at catching highly unlikely conditions to protect themselves against the hypothetical lawsuit that asks "why didn't you test for that?" Whatever the truth is about malpractice suits, the perception in the industry seems to be that these cases are overwhelmingly biased against doctors if even one question can be raised about the course of treatment.