The resistor or NTC limits the surge current to a value that is tolerated indefinitely by fuses, diodes and switches used in the PSU - ie something like 20amps peak for a couple of milliseconds.

I think that since the surge current occurs during the peak of the 120V 60Hz power waveform, I think the timing is more like several milliseconds or a few 10's of milliseconds at most, but not just a couple of milliseconds.