It's basic market segmentation. They recognise that they can get business travellers to pay the higher prices so why offer a discount and impact yield.
Basic segmentation is more damaging than micro-segmentation for many companies.
Let me explain. Targeting companion sales instantly alienates a number of audience segments, the biggest being self-funded single travellers who buy on value and not price.
Qantas, like many airlines, know what pax have historically paid for a ticket, and have conducted very basic tests pricing tests on a large scale.
What they don't know about customers is the most damaging, especially as more and more generic campaigns are placed in the market.
Qantas holds very little intelligence on what each pax would pay for a particular flight.
For example, yo may have a bne-syd/syd-mel budget of $400/flight. If business class, available at this rate - great - You'll book that. Otherwise, it reverts to the cheapest possible ticket available which may be $180. Effectively meaning, if business isn't full - it's a $220 (difference in price) + $180* (re-selling the original ticket) = $400 (100% of opportunity cost to the airline). Extrapolate this further, and it explains why we see so many program splitters where the price can be a determining factor.
This is why airlines are rushing to do personalised pricing. So the price you see will be what the airline thinks you will pay, rather than a ridiculously crazy 'market rate' which is a turn-off to many audience segments.
Let's not get started on award tickets. Seriously under-performing segment for Qantas.