they were operating on spectrum outside of the normal “blocks” allocated to their cities (e.g. in Melbourne ABC, Seven, Nine, Ten and SBS were on a VHF block, C31 was on a separate UHF channel).
They could easily have been assigned the existing 6th channel that has been set aside in each market, and C31 in Melbourne has said that as far as it knows there are no immediate plans for the “borrowed” spectrum they were using to be re-allocated. So there is literally nothing to be gained by shutting it off except, as TVHead says, the government just wants to knock it off.