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Why do banks insist on allowing transactions without sufficient funds?
This really should be a comment, but I can't yet. The question desperately needs a location tag. In at least some countries(New Zealand), the default action on all insufficient funds transactions is to refuse the transaction. Credit cards are the only common exception. Every bank operating in NZ that I know of acts this way. Sometimes there is a fee for bouncing a transaction, sometimes not, that depends on the bank. Any other option must be explicitly arranged in writing with the bank. Personally, coming from a country where declining transactions is the default, I'd be shocked and angry to be stuck with an automatic transfer from another account. Angry enough to change banks if they won't immediately cease and desist.