500' aal would make a great deal more sense but I suspect that FLIDRAS cannot really measure that and so the insurance companies look for 500' RAD ALT. Get enough approach's stable and your premium goes down.
Clearly the line has to be drawn somewhere. You could call it 100' and the same argument would apply ie is an approach that gains stability at 99' any more unsafe than one that gains stability at 101'.
I rather suspect that common sense is applied and the whole approach is looked at and environmental factors are taken into account as per SOP.