It's disgustingly arrogant to assume that. They are showing exactly the understanding needed to answer the question. They obeyed the instructions, and they paid good money to have that opportunity, but they are being punished despite doing exactly what was asked of them. Whether the instructor has the legal right to do this or not, it certainly feels like fraud.
If the question setter failed to set a good question, that's the question setter's fault, not the student's fault. Punishing the student for somebody else's mistake is injustice.
If the question setter failed to set a good question, that's the question setter's fault, not the student's fault. Punishing the student for somebody else's mistake is injustice.