If there was no cost to offering a priority checkin desk, then we could offer it to everyone, and it would cost the airline nothing. But obviously that's not the case.

Priority checkin generally implies having a faster service. If we know that only 10% of people fly on full-fare/starclass fares, then Jetstar can make desks available as appropriate. If we then add another 10% of people who are entitled to priority checkin by virtue of QF status, then that would (to keep the service level the same) involve opening and staffing double the number of checkin desks.