Since when is everybody with a piano degree a professional pianist? That seems unduly restrictive. The Cliburn amateur competition only says "Applicants may not derive their principal source of income from public performances or piano instruction. "
What is the percentage of professional pianists who do not have piano degree?
It is totally making sense. What is the reason people went to, say, a nursing school? To practice as a professional nurse. Or why people go to a law school? to be a professional lawyers.
Now let's ponder why people went to a conservatory? I think most people went there to have a credential so that they can make money either from teaching or performing. Or do you think people pursue a degree just for the sake of having a degree? I really believe people getting a piano degree, because they want to make a living as a professional in the piano field. However, later they may decide to make money or not is their choice, but by trade they are a professional.
Therefore, I think the Van Cliburn people need to add one more qualification in their application form stating that a professionally trained pianist should not join the competition.