You didn't ask this, but it's possible that such behavior could violate your school's grading policy, regardless of the ethical and legal concerns. The central problem is that grades absolutely do matter to students, and they have a reasonable expectation to understand how they will be graded.
For example, the first google result that came up for me states:
Instructors are obligated to evaluate each student's work fairly and without bias and to assign grades based on valid academic criteria. (my emphasis)
To be frank, there's a lot of fuzzy and subjective reasoning that goes into grading, and it sounds like this professor's subject might incorporate more of this than usual. Actually finding an administrator who was willing to do anything beyond having a talk with your professor (i.e. formal sanctions) is going to be nearly impossible. Typically the notion of "bias" in grading policies refers to grading some students easier or harder because of discriminatory characteristics like race, gender, or sexual orientation.
Also, your professor's grading might be based on perfectly objective and fair criteria that is not the overall quality of the student's work. For example, a workshop class whose purpose is to refine student's writing style or skill is going to focus heavily on the revision process. The point here might not be to produce masterful essays, but to critically examine and improve your own work. A great essay with minimal evidence of revision is not going to score well in that kind of situation.