Large amount of overages can still occur for customers of other prepaid carriers, or at least it's possible. But the reason for it never getting that extent has to do with those charges would be limited to the current balance topped up in the account. I can't see someone topping up more than a few hundered dollars at a time, and even then, customers would probably only do that because of a special sale price on prepaid vouchers, or if someone without credit cards wishes to eliminate the need to top up every billing cycle.
In the case of Public Mobile, it can be a bad thing that there are no overage charges, because once everything included in the plan or add-on is used up, everything stops working. However, that can also a good thing because on Public, short of a billing error, there are never be any surprise charges.
