Ok, I think we can ignore the issue of 'what about the design E motors?'
The way I see it, it seems to come down to a question of why they don't include 'design A' motors. From what little I've gleaned about the subject, design A motors don't have a defined inrush draw. They are a sort of wild card, as far as design is concerned, with no limit as to the inrush current it might draw. It presents a problem because, as I read somewhere, you have to install it and see what inrush you get before you can properly size your disconnects, etc. ..which is a bit unrealistic. Anyway, since design B and C and D have a constrained and characterizable inrush, then the NEC can safely specify that it is ok to use 75C ampacity wiring, assuming that the 75C will be cooler than the motor leads (which we all know are usually very hot), and so won't lead to overheating the connections, etc. Of course, there is still the question of what happens at the source end termination.