That means they have to put staff on all flights. I'm sure someone did the calculation and what they do today was the most cost effective.
That's what I'm saying--put a higher cost on running empty planes. If they have to give away 100 tickets (or pay passengers to fly!) and staff the plane, they might start wondering whether it's worth it. Also that kind of giveaway would deflate prices across the market, making this empty planing a ruinous exercise for everyone.
So the airlines will start hire passengers :)
Lufthansa is pretty far away from crossing the $0 threshold.