I agree that some readers definitely do more harm than good. I don't think they could sign an oath to do no harm though as there is no standard for psychics as they are classified as entertainment only. Doctors and psychologists have standards that they follow for their licensing. There are protocols that have been established through testing to be the most helpful and appropriate for those professions. Since psychic abilities are seen as fake by authorities and perhaps the public at large, there isn't much criteria to go off of. Psychics have to follow the rules of the platform they are working on, but beyond that, each psychic seems to have their own way about how they do their readings. They also seem to have vastly different levels of skill, and just like doctors (who at times get charged with malpractice), even the more successful ones have their off days.
I just try to not take predictions too seriously since no psychic is omniscient. Even if they are good, they are still human and are not flawless. They may just have that bad day when I come for my reading.