And if quantity and type of receptacles are intended to be a form of overcurrent protection, wouldn't the UL refuse to certify a duplex 5-15R receptacle for use

And if quantity and type of receptacles are intended to be a form of overcurrent protection, wouldn't the UL refuse to certify a duplex 5-15R receptacle for use on a 15 A circuit? The fact that they do not leads (non-electrician) me to believe that outlet types differ to prevent unintentionally plugging devices into inappropriate circuits, not to prevent intentional abuse. The present example is just a band-aid fix for cases where lower-current devices are connected in through some intermediate wiring (UPS, extension cord, RV power system, etc.) with a plug consistent with its own, higher ampacity, and in this application, seems no more prone to abuse than a garden-variety 15 A duplex receptacle on a 15 A circuit.