this is what I found in the Medicare Claims processing Manual. It seems to suggest that If you have two patients with the same condition requiring a certain treatment, you cannot offer only to commercial patients and not Medicare.
Medicare Claims Processing Manual
Chptr 1 30.1.3 Provider Treatment of Beneficiaries In the agreement between CMS and a provider, the provider agrees to accept Medicare beneficiaries for care and treatment. The provider cannot impose any limitations with respect to care and treatment of Medicare beneficiaries that it does not also impose on all other persons seeking care and treatment. If the provider does not furnish treatment for certain illnesses and conditions to patients who are not Medicare beneficiaries, it need not furnish such treatment to Medicare beneficiaries in order to participate in the Medicare program. It may not, however, refuse to furnish treatment for certain illnesses or conditions to Medicare beneficiaries if it furnishes such treatment to others. Failure to abide by this rule is a cause for termination of the provider's agreement to participate in the Medicare program.