These customers did not fall behind. However...it's more than just what you posted - the bank is entitled to all of your money at any time as it is not actually your money. You give the bank a loan when you deposit and you are considered an unsecured creditor. As you say, it's in the fine print.
As for your second point, you are correct but...there's nothing to stop your deposit account bank doing the same thing when they feel pressure, as per my previous paragraph.
There are two types of home loans here. As I understand it, there are a small group of people who hold legacy facilities with ME Bank where their home loans are essentially an interest only line of credit.
This is different to an off-set account which sits beside the P & I home loan account and the interest that could have been earned on savings is "off-set" on the interest charged on the mortgage debt.
I used to have such a facility with St George and it was called a Portfolio Loan. So you would essentially pour all of your cash into the home loan account and you would get charged interest on the debt only. St George tweaked how these loans loans worked through pressure from APRA as it was viewed that loans were never being repaid and national household debt floundering. First they said that you must deposit at least the loan repayment amount into the account each month, irrespective of how much you are in credit. Then STG stopped offering the product in that format.
I think this is where these small group of ME Bank customers are being tripped up. They have not been paying regular payments into the account as they erroneously thought that being "ahead" of the loan meant that you did not have to make regular repayments.
So again, I think that the reports are not being entirely transparent with what has occured. I suggest to you that the customers have been "behind" in their loans as they have not made regular contributions to the home loan account.
APRA have turned the knob on household debt and are trying to squeeze out these types of interest only facilities.