The reason why you cannot reverse an application for a customer refund is the same reason why you cannot save a customer refund that is not associated to any Credit Memo, Payment or Prepayment as shown below.
Customer refunds must be attached to either a Credit Memo, Payment or Prepayment in full or else the system design will view the document out of balance. When you are reversing an application be it a credit memo, payment or prepayment against a customer refund you are unattaching it from the customer refund therefore making the document out of balance - think one sided journal.
