I completely agree with you. I remember when I was taking my Toefl, the speaking section consisted of texts related to:
1. Feminism
2. Black people
3. U.S. Psychology
The first two were quite easy to regurgitate since we have seen so many films and have red so many books on such topics, but the third one was really difficult to interpret simply because I have never been introduced to the U.S. psychology before. I have noticed that every nation does the same thing actually, for example, when I was studying English from a British textbook, we was reading nonstop about drinking tea, dining or waging wars in the 15th century and sending prisoners to another continent (probably meant to be Australia) and they didn't find those topics boring or racist.

If I was a creator of Bulgarian textbooks, I would have included chapters describing what a Shopska salad is or providing a chapter dedicated to the national Bulgarian heros such as Vasil Levski or Hristo Botev because their American counterparts teach us about their national heros such as JFK or Martin Luther King Jr., etc.