Probably so they can screw down the T & C with Jetstar then go to mainline and tell them to take it or leave it. However given what is going on in the pilot labour market at the moment Jetstar might be struggling to find people to fly the aircraft at the wages they want to pay. I'm sure they could find plenty of emirate 777 people if they were prepared to pay the money.
There was also a interesting post around here in regard to Jetstar's lack of maintenance planning for the 787.