it's a very simple task to limit engine power to some idle while the mechanism is in transition
the problem is the reliable determination that the reverser is in transition.
One little bobble on take off would be very very awkward.
Software is easy and the reliability of FADECs are adequate to do the job. It's the reliability of the command and motion structure of the reverser that precludes idle until you are absolutely sure that the idea is to stop and not to go.