Adrian Yeah this would be pretty nice, I agree. I'm not sure if bunq is allowed to do this, though. With IBANs, while they have a country-specific code at the beginning, there's no law or contract that is stopping bunq from giving a Finnish customer a Spanish IBAN, and so they do it. But when it comes to the Mastercards, they have to uphold their contractual obligations with Mastercard. Which I don't know about, but it's likely that they wouldn't want just anyone getting a card from a random country.
But, I admit I could be wrong here, and maybe they just didn't bother with this because it's kind of unintuitive for most customers. They probably don't even know that there are different BINs for the different countries and some stores like iTunes as you mentioned or Netflix are using these to check if the card can be used. Exposing more options in the app always has the danger of overwhelming users / giving them more trouble than if you just make the most obvious choice for them without asking. As you see, even a power user like Thijs didn't think about the country BINs.
It would also make it probably possible to use Apple Pay and Google Pay for users in countries where it's not available at bunq. This would be great for users, but there ought to be a reason that it is like it is right now, so maybe also it's not possible because of the contracts with Apple and Google.