I posted in an earlier answer that it was possible that neither your son nor the bank person that suggested the credit card knew how the credit card thing was going to play out.
Focusing (wink) on that I think I may have an answer, and it's not a business ethics (?) its what I call an
IT Ethics question. Let me explain.
Products are issued by banks with 'X' as the objective. That is in this case (I hope) to lessen any overdraft charges. However, during the IT/posting process the IT portion recognizes the overdraft as what it really is..an overdraft. The IT in effect due to the way charges are processed 'bypasses' the 'intent' of the linking of accounts. When new products are offered,
IT changes are not made to deal with the new 'what if X', 'then this Y' aspect of the financial. product. In other words, the DomainExpert of the financial institution may either not have been notified of the new consumer product, or was in the process of making any changes to the domain or was prohibited by Sarbanes-Oxley acccounting rules & regs from doing so.
Take a moment and digest my experiences (2 of many) with banking IT and account linking.
While banking at another institution now seized by the Fed I constantly argued with them regarding charges. My accounts were supposed to be 'linked', mortgage, business accounts, personal accounts so that when the total sum of X was reached, all service fees were waived. It took eight months of me going to the branch every month, get the service fee credited in. Each month the branch manager would call the help desk, the last time he called some IT guy answered and he finally got into a part of the system to send an email to IT to tell them to LOOK at the account list under my social and to get the links straight.
Banks are notorius for compartmentalizing their accounting. I have actually recieved call from collections regarding fees on lines of credit that are waived because of my account links and status. Even the branch manager and the executive help desk were not able to stop the calls. The calls stopped when I was finally ALLOWED, yes ALLOWED to speak with the supervisor of collections who then went into 'another part' of the system, and read all of the 6 weeks worth of comments and then removed my number from the automatic deliquent account call list.
The problem as I see it, is that the institutions 'Domain Expert' is out of touch with the actual 'consumerism' of the financial institution.
If I had not followed up, and stayed on it...I would have been out hundreds of dollars of fees. All banks have their particluar IT quirks. FWIW, I still think you dropped the ball in assuming your son knew the 'accounting stuff' however, a trip to some BofA Presidents office may get you some relief and call attention to some IT problems they may not even be aware of.
Good Luck. It can be done.