I suggest you ask to see their policy and procedure manual so you can familiarise yourself with how and why they do things. There will be a section in their on nutrition which will explain the rules their centre follows, the manual should always be available for parents.
Also look up the acecqa website for the national standards and regulations. Find the ones regarding nutrition and you should have your answer.
I've written a lot of policies over the past few years and I've always written that we provide milk and water and, on occasion, juice - the juice should be watered down though and children are encouraged to rinse their mouths after drinking juice.
It's a long document but once you find the area you're looking for you will have your answer.
But as I said, if the centres policies and procedures state that they don't allow it, then that's their rules and you will have to follow or find another centre.