They really are letting the milk go slightly rancid as a method of preservation. Which is why their chocolate tastes like rancid milk (or vomit).
Now I understand that those who grew up eating it are just used to the flavor. The real mystery is how it became so popular in the first place. How was vomit-flavored chocolate preferable to not having chocolate?
I would think that giving people care doesn't actually cost much, it's having the capability that's expensive. And the administrative work required to deal with the edge case of charging foreigners might not be worth the minor sums involved.