It's being paid for...
...by allowing them to loan on my money to other parties and making a nice chunk of interest out of it. Also I'm sure my money (and others) appears in some line in their yearly reports, indicating some form of success in their business and encouraging others to invest.
And I allow the banks to do this with my money for free. So if the bank wants to make it easier for me to transact my money with them; don't be taking the piss and try charging me for it. After all, a house safe is free, available 24 hours and a lot safer (pardon the pun).
I had this exact conversation with my bank manager 25 years ago and have not been charged any transaction charges since.