Buy it at all? Do they have to grow their own?

While I agree with the sentiment feel the urge to note that where I live but you will face punishment for having blackberries growing on your property similar to what you would for selling marijuana.

Blackberries are a national pest, and yet sell for $80/kg in supermarkets. The average person could collect over $50 worth from the side of the road in under an hour.

Australia is not known for its efficiency nor logic.

Not legal, merely “tolerated” (gedogen) by the state and their agents.

It is illogical, but allows policing when things are perceived to "get out of hand" which is an interesting way to have things both ways (both as citizens, and as law enforcement). It's a delicate balance that could be easily thrown-over, but so far has managed to last. We'll see how long it can continue though given a lot of boundary-testing.