This is a good point. Doctors can prescribe drugs approved for disease X off label to treat disease Y even when effectiveness against Y has not been proven. In this case, the drug has not been approved for any disease so it can't be prescribed. Just goes to show how arbitrary the rules are.

I'm not sure that makes it "arbitrary." Approval is not just about effectiveness but also risks and side effects.

If we've gotten to Phase III, the risks and side effects are understood as well as they would be for a doctor prescribing off label. While I might not call it arbitrary, I would say it's an unappreciated edge case.