Isn't it clear they're doing it to be consistant with their International operations (eg: transferring pax from International to domestic etc...) That's all! Imagine a non-english speaking tourist group flying from overseas with allocated seating then transferring to a domestic flight without allocated seating but being the same product/airline?
“Seat allocation will form part of Jetstar’s in-flight product for our low fare long haul international flights which will also feature two classes of service."
“Jetstar will in three years be a predominately international business and consistency of an allocated seat for all Jetstar services is fundamental to our product offering."
MIss Behaviour...
It's only a matter of time before the scenario happens, so taking the above into account are the cabin crew encouraged to have a bit of a rethink or does one pax ruin it for everybody by having 170 people diverted somewhere else (less crew hours) with the extra expense of overnighting pax in hotels then still having to get them to their original destination?
Not sure about Jetstar (presume they're the same), but we as cabin crew can be rostered up to a certain amount of hours work, add another 1.5 hours for ANY delay. So I don't think your scenario should be a problem.