I've trained on Cricket fields for RL all my life. Down here in shellharbour the local teams use about half a dozen fields for their juniors to train on, 3 or 4 which are cricket ovals in Summer. I'd assume Sydney would have at least a similar kind of demand for space.
But I think the main uproar about this is the fact anything being played on this oval needs AFL approval, which would really be limited to Aussie Rules matches/training and School Auskick. Even if a kid wanted to go down and practice his goal kicking they probably couldnt, whereas I've always been able to go down to Ron Costello and kick some goals, even though the only organised sport played on there (as far as im aware) is RL. And Im not even too sure of the grounds relationship with the council, as I know when I played we could play there when all the other council grounds were closed due to rain, but I think the club had to maintain the ground. No idea what the deal was after they got admition to the NSW cup aswell.