Should non-DRM players return a serial number when queried by the host?
No. If a valid serial number is returned, the WMP (or any other download software supporting WMDM) assumes that the player supports DRM; so, if the transferred file is DRM (v1, v7) on the PC, it will be transcribed to DRM-PD and tied to the serial number of the portable media. Clearly such a file cannot be played on the player that does not have DRM-PD implemented on the player.