If you want to argue who sells what to who, when and where I'd suggest getting in touch with Apple. Perhaps they'll be able to add your card to your account if you explain the situation.
I'm not, however, going to defend the position, but I stand by what I posted earlier. The whole reason there are different, per country, iTunes stores is because different countries have different legalities and not all content is available in all countries under the same licencing/rules/regs etc.
If you are in any way unsure about buying an iTunes card to fund your account I would again contact Apple support directly.
And in about 28 years of computing I was never restricted to only one supplier. It was always possible freely to choose from who and where I bought hard- and software, even abroad.
In this case that's not the issue. A supplier of any kind of product chooses what they believe to be the best outlet for their product. It's not uncommon for a supplier to make a product exclusive to one store, bricks and mortar or digital. It's all the same.
When Dell first started selling Dell PCs you could only buy direct from them. One single supplier.
I'd suggest that if in 28 years you haven't come across this kind of selling it's just been your personal choice to compare and contrast again products sold by multiple vendors.
You
can buy Mac software from all sorts of places. You aren't restricted to the Mac app store.
It's just that with this particular product the supplier has choosen to sell via a single outlet, that's up to them and is a completely different issue from that of using a cross-nation credit card for purchasing.