yep understand yield. You are talking of extreme loss leaders like $1 fares, which are basically put out to get free advertising.You do not understand yield. I suggest visiting Google.
A fare sold is not always better than it going empty. You forget that weight not used in the passenger cabin can be used below as cargo which may work more favourably for the airline.
In most cases freight generates less revenue per kg, plus think freight revenue is falling faster than pax revenue