FNB allowing late deductions on accounts

Why is FNB allowing merchants to do as they please on our accounts? I swiped my card on thursday from my cheque account and the money was only deducted from my account late Sunday same week. This is a serious inconvenience to its clients which could have an impact on their credit score if there are insufficient funds on the account. I have banked with other banks and this has never been an issue. I tried speaking to the team of bankers and all I got was excuses as if I use merchant bank cards whilst I am using FNB's card.