You're making the mistake of assuming our FDA makes any sense.
Because fiber is chemically/structurally a carbohydrate it is listed on the nutrition label as a carbohydrate. The fact that it has calories at all is what makes it necessary to include fiber on the nutrition label.
Think about if you were making alcohol. You would start with a sugar (carb) but then the bacteria would break it down to alcohol (still has calories, but not a carb) and the process would be finished.
So even though it is absorbed as a fat, fiber itself is a carbohydrate and listed as such.