I've never done one of those -- how much information (and why in the world would they need it?) do they ask for just to act like an ATM for you? The PIN number should be the only identification they should need. Maybe a glance at a driver's license to be sure it was your card.
What did they ask you to provide? That just seems bizarre.
I'll be sure to move most of my money to the savings account first before I try one of these shops. I do a lot of shops at those places but haven't done a debit withdrawal. Of course, I have to wonder why anyone in real life would have to withdraw money from one of these places instead of just going to a bank ATM.
I wouldn't worry too much about them knowing your balance. They have no reason to care about it. For all they know, most of it went out the next day to pay your property taxes or mortgage or your kid's college tuition bill.
Time to build a bigger bridge.