I am not surprised that newer folks in the trade (LV, A/V or electrician) believe the factory is the only one that can install a plug. We heard the same thing about plug caps on line cords (loss of U/L listing prevents a field installed plug).
They also have probably had problems with signal cables that had field installed ends. I know I would never try to make up a USB cable unless it was an emergency. Maybe I would have when my eyes were 30 years younger.
