[neilcrj]

I have a standard rubric for questions like these, and would have scored the response above 1/5 for an attempt being made with an incorrect or unclear argument. My expectations are very clear.

I disagree with your characterization of grading as punishment. Added value in teaching is about 10% instruction and 90% feedback. If I let this opportunity to fix the student's reasoning pass, I am doing a far greater injustice to them than if I fail them on a quiz.