Power supplies can be tested separately from devices. That is a major driving force behind the fact that almost every device has an external power supply nowadays.

Also, that "it runs on 5v DC and has not PS it should be OK" likely is true, but somebody has to tick some boxes to confirm that the device is safe. And no, that is not a purely bureaucratic thing. For example, there are 5V driven braille displays with piezo actuators that internally produce 200V+ voltages, and could be dangerous. That voltage is present centimeters from the fingers of its users.