Originally Posted by
NewYorkMoscowFlyer64
The fact is, Delta purposefully does not release BE seats into Z class because it wants to save those seats to either sell at an even higher price or to use in case economy oversells.
Releasing a BE seat into Z doesn't hurt if economy oversells; either the Z seat is taken (via SWU/miles) by somebody in Y, freeing up one economy seat, or releasing it has no effect.