Since I don't know this particular story, it might be a story that is specific to a certain community or area. It could be about a relationship between a Kabuliwala (someone from Kabul perhaps) and a Bengali woman. But without more information, it's hard to tell the full story.
Typically, 'bangla premer golpo love story' might feature strong family influence. In Bangladeshi culture, family plays a big role in relationships. So, the story could have elements of family approval or disapproval of the love.
Family values play a significant role as well. In Bengali golpo love stories, family approval or disapproval can be a major factor in the development of the love story. Sometimes, the characters have to fight against their family's wishes to be with the person they love, while in other cases, they abide by their family's decisions which leads to a different kind of love story, often a tragic one.
One possible theme could be the fear of the unknown. Since it's a horror story, the 'bhuter' (if it's a supernatural entity) represents something that people don't fully understand.
I'm sorry, I'm not familiar with the specific 'kabuliwalar bengali bou' story. It might be a relatively unknown or regional story. You could try to find it in local libraries, ask local storytellers or search in specialized Bengali literature resources.
Unfortunately, without more context, it's difficult to provide the full story of 'bou kotha kou'. It could be a name of a story within a small community or an emerging piece of literature. If you could give some more details like where you heard it from or what kind of story it is (e.g. a love story, a mystery) it would be easier to answer.