If the power supply is being powered from the incoming 120V, that is what they are looking for, the connected load is immaterial. The purpose of the rating on the nameplate is to ensure, at a glance, that the circuit feeding it is sufficient for the maximum possible amps that it will theoretically be able to consume from the upstream circuit, as defined by FLA ratings of all devices inside. What the devices inside or down stream actually consume does not figure into it.
To further the example, let's say you had a 480V feed and a motor starter, then a 480-120V CPT, then a 120-24VDC power supply. The nameplate data would be derived from the motor FLA and the primary current of the CPT. The power supply input current is ALREADY going to be part of the CPT circuit current.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
� Kilgore Trout (via Kurt Vonnegut)