While there is some additional cost to Qantas, it is tiny in comparison to the benefits they get from having more flights booked with them.
Think about it ...
1. They don't have to hire more lounge staff, they just make everyone wait a bit longer for their coffees in the morning, their beers in the evening and their food in the First lounge.
2. They don't have to make much more food because buffets already have excess built into them (they are terrible from a food waste point of view). We're talking a couple of extra beers bought at wholesale discount prices (eg $1 per beer).
The only substantial cost Qantas faces is paying for its own frequent flyers to access other oneworld lounges. But realistically, how many times is that going to occur? Remember we're talking about people who need DSC offers to get from Silver to Gold and from Gold to Platinum, so they are going to be trying to direct as much of their spend to Qantas, not other carriers, as possible to get them over the line.