Think their aim is for all food to be bought
I have worked in the medical tent for quite a few of these events. It is absolutely a safety issue. The camping at these events is not well organised and most of the attendees are not seasoned campers and many are alcohol and/or drug affected. The camps are crowded with poor access. It can look like a shanty town and the tents are so close to each other that fire will spread quickly and evacuation will be difficult.
The organisers of these events are professional (it's a big business) and paranoid about safety. There have been quite a few deaths at such events recently (generally OD's) and before local government issues a Public Event permit Emergency Management planning needs to be tight. The bigger events even have private fire crews with fire trucks on site for the duration of the event. At the last event I attended there was one camp that kept re-lighting an open campfire after they were told to put it out repeatedly. After their third cycle of being warned, putting it out then re-lighting the fire the local CFS put it out for them as only a fire truck can! (The fire brigade auxiliary were running a food stall at the event)
Your daughter may be sensible and an experienced camper but unfortunately the rules are for the idiots out there. You will sleep better knowing that.
ps food prices aren't too bad at the multi-day bush events and you may even be supporting the local fire brigade. I think it's the one-day events in the city that tend to be expensive.
pps electrical cooking is not really an option as generators are usually banned.