JMich: Small correction.
Bank and Card specific, not Visa (or Mastercard) specific. I had a card from one bank decline a payment to IndieRoyale because it goes through Australia, while the same card works on GOG just fine, and a card from another bank has no problem with either one. It is usually a case of allowing foreign transactions or blocking specific countries, for "Reasons".
Total error on my part that needed correction. Thank you.
Orlim: I am still baffled. I am a dual citizen of the US and Germany and have accounts in both countries. I have never ever had any kind of payment issues with Chase, Bank of America, Postbank or the Sparkasse.
I was also not aware that Visa or Master were nationally supervised. They are global traders with their own fraud detection tools. I would have run in to that issue with my American MasterCard at least once, no?
While not in a way that is relevant to the discussion at hand, yes both Visa and Master card are in fact nationally regulated where applicable. Just getting that out of the way.
Related to this issue, as Jmich explained it is bank and card specific. The assumption that you would have ran into in your situation is incorrect and really out of place as no one has suggested that all banks operate identically or interact with payment facilitators such as MC or Visa in the same ways.
There is where the confusion begins. Understanding what credit cards like Visa or Master card actually are. Visa, MC, Discover or the like do not issue cards, establish credit, set fees or policies. They are payment facilitators. All they do is interact between bank and merchant. They "facilitate" payment transactions between a merchant and the respective bank. Sort of like the doorman deciding who gets in to your bank account. All the little logo on your card means is that your bank facilitates transactions via MC or Visa, or who ever they put on the card they issued you.
This is why in the US you can have an account with Chase, that comes with a Visa card, despite the fact that Visa is a subsidiary product of Bank of America.
So when your original point was on being upset over an assumption of the purpose of this was contingent on effectively "I don't get it because adults can get real cards" It becomes important to understand why just because you have an "real" credit card does not by default mean that you will have the capacity to utilize it where you wish.
From GOG perspective, it falls to one of the oldest and golden rules of commerce. The easier you make it for customers to buy from you will result in customers buying from you more as well as more customers buying from you. Even if those customers are poor kids as you seemingly take umbrage with. Just because GOG is for doing something to help their customers, there is no rational reason to get upset because it does not apply in your personal circumstances.