......but from a business perspective it is much better for QF to fill those seats for cash or points than to have them fly empty.
I think this is where QF yield management disagree with you - and is at the crux of the different way the US airlines look at things.
When looking at the "business perspective" you need to consider both short and long term. Your assertion is certainly true for the immediate short term - filling the seats with cash or points gives a better immediate return for that flight than letting it go empty.
But what about the longer term? By allowing "cheap" (points or cash) sales of the seats, you then start to raise questions in the minds of those paying the full price - should they also try for the cheaper option (buy Y, look for upgrade). The greater the chances of success (i.e. the harder the airline works to not let empty seats fly), the greater the incentive.
The maximum yield from the seats in the cabin is the total gross paid - all the seats full at a lower price is less profitable than half the seats empty, but the remainder sold for a total price greater than that obtainable if they were all sold at a discount.
Obviously this is simplistic, and there are all sorts of considerations about loyalty, giving people a "taste" of the higher cabins, etc., but that is why Yield Management is such a dark art.
Sufficient to say, it is not at all true that it is always better (in the long term for the airline) to sell a seat for points or cash rather than let if go empty.
