People may prefer a foreign bank for any and every good reason besides mobile OS nerding, and that's nobody's business. Certainly not your employers business.
As for BankID, I know of it and it might just as well be called ThrallID from how ridiculous and pathetic it is that people would let their IDs be controlled and owned by private banks. You must admit that it's nothing to be proud of.
You can also keep your national bank account for your salary and ThrallID if you want to, and just transfer all your money to a foreign account every time you get your paycheck. That way your money is also a whole lot safer, and you get some better peace of mind.
Also, theory and practice are very different. While I agree you can fight back for your rights, I remember having contacted different kind authorities for 7 months because a company was rejecting my SEPA IBAN. Unfortunately, that was the only company I could use for that mandatory service, and after they got fined, they happily continued accepting only national IBANs.
The result has been months of frustration, endless calls/appointments with authorities to gain nothing. If I could go back, I would give them a different IBAN, even if on a theory level they were wrong.
Moreover, you aren't considering that some countries have BankID systems for everything from tax matters to accessing healthcare, and that only works with a local bank account (so much so that people moving from other EU countries are commonly shut out of everything for weeks until their local bank ID comes through).