[KVFinn]

>What I heard was that payment providers charge higher fees for allowing foreign cards, so website owners (who focus on domestic business anyways) just won't bother.

Here at least that doesn't seem to be the blocking issue. Or at least I often see foreign CC payment options in the list, but if you try to use the option you still need a Korean phone number in the checkout process (and they confirm with an SMS, you can't just type in whatever.)