I still think we don't have a reasonable explanation of that provision, but I am inclined to believe that it is most likely driven by bankruptcy law.
I know that a plan can be crammed down over a dissenting class of creditors, but only if it can be shown that the dissenting class will do at least as well under the plan as in a liquidation.
However, I don't know if that rule applies to a dissenting class of senior equity; maybe it does, and that is why the plan provides that if the preferred class dissents, then commons get nothing, which would then mean that the dissenting prefs would do no worse under the plan than they would under liquidation.
Just an educated guess.