Is it acceptable for a doctor to drop a patient that they are seeing in retaliation for having filed a grievance against them?

Physicians are not obligated to see patients for a variety of reasons. There are protocols to dismiss a patient to avoid abandonment. If a patient has filed a grievance against their physician, it is assumed that the patient no longer wants to be treated by that physician. Dismissal is a formal way of severing the relationship.

It has to do with trust and giving the patient a chance to have access to a second opinion. For as long as this is well explained, it is not retaliation, it may be in the best interest of the patient.