I suspect that these "power supplies" are actually UPSs and the 13amp is their output rating.
If this is the case, they need a 20amp plug because they can charge their batteries AND provide 13amps to a load at
the same time, this puts the input over 15amps!
Hence, the 20amp plug to achieve UL listing.
Correct outlets on proper circuits should be installed.
Alternatively, get smaller units that will run on a 15a source.
Shame on those who ASSUME that the plug is wrong and suggest changing or modifying it.
