Catering is provided by an external company. I fail to see how Jetstar is able to find a profit there!
There are many possible arrangements but likely,the external company supplies food and drink which is sold inflight on a sale or return basis with the operator taking a percentage of sales.
How many petrol stations would stay in business if they could only sell fuel ? The high margin convenience store items provide the profit while fuel sales barely cover the overhead. That magazine, packet of cigarettes, bar of chocolate and can of coke you bought last time you filled up keep the business in profit.
Repeat after me "If we fly twenty million passengers a year and we can get another $5 from each one that's $100 MILLION."