The circuitry that converts the 120 VAC to 5 VDC is not in question. Agreed, the charging circuitry is not a receptacle. The part that the plug goes into is. That part connects the plug to the charging circuitry which is permanently attached to the device (USB receptacle).
Just could not resist logic chopping a bit more (and thanks to K8MHZ for the reminder to be careful about outlet versus receptacle). If nobody cares about it here, we can continue this on the Puzzles and Illusions thread.
Is the USB connector a device in and of itself? If not, does the device include the USB connector, the power supply and the yoke and 120 volt receptacle, or just some subset of these? It is sold as a single inseparable assembly.
If the device is the power supply plus USB connector, I would still argue that the USB connector is not a receptacle in the NEC sense because it does not interconnect its wires with external wires attached to the device. It connects only to components in the power supply which is contained in the device.
If you consider the device to be just the USB connector then it is not installed at a point on the wiring system where current is taken to supply utilization equipment. It just takes current from the power supply, which is connected to the wiring system but is not a point on the wiring system.
In any case, we do not have any good guidance that I can see as to how many devices are actually part of the assembly. And in particular as to whether a part of a device can be a device in its own right.
I do not declare victory, but I think I have reached the limits of my interest in this subject.