Galdian - G'day!
I am inclined to think the substitution of JQ for QF would have happened to passengers who had booked pre December, when the service dates of the QF3/4 ops were changed and mixed with JQ. At the time passengers who booked with humans were given the option of swapping to the nearest date with QF metal or remaining on the QF codeshare on the JQ metal. This was generally without compensation as most of the fares at that stage were fairly close to the regular JQ levels (opening specials notwithstanding of course).
Those who had booked on the web would have received a schedule change notification detailing a QF300 series flight number and modified timings, but would most likely have not twigged the "operated by Jetstar Airways" disclaimer under the flight details. Hence, they would most likely have gone wandering happily to the 'drome only to find an A330 parked where they expected to see a 743 or 763.
The QF codeshare on JQ can actually be sold on a return basis providing one leg is on QF metal as all QF flight numbers through the GDS's. This is done on QF published or nett remit fares.
Assuming the same occurs with web bookings, many would be unaware that they have booked a JQ seat in one direction and that would mean they have to read.
As for litigating - read the terms and conditions of travel.
A basic simplification is "It's our aeroplane and we can do anything we want".
Hope this helps
Best regards
EWL