BoA doesn't have any transferable currency though, right?

Of the legacies, UA partners with Chase and Bilt, AA partners with Citi and Barclays (and Bilt for a little while longer), and DL is the only one going it alone with AMEX. It's not unheard of to have multiple banking partners. Especially when your banking partner, well... isn't exactly known for its stellar competence or customer service and might be a reason why your customers don't want the card. BoA is fine until you have a problem and then it's like pulling teeth to get it fixed. Chase and AMEX are no picnic but I've never had anywhere near the amount of problems with fraud with them that I've had with BoA cards I've held, including the Alaska card.