A 3 litre keg, which costs $28 to refill?
3 litres is equivalent to 4 longnecks., so that's $7 per 750ml longneck.
More expensive than beer in normal bottles or cans. Even more expensive than a Coopers longneck, and there you get to keep the empty for your homebrew.
What are they worried about? Oh, I forgot, publicans can't do the math. Can't see it being taken up by too many.