This doesn't make any sense. If it's BMI based, then those who have higher BMIs would have liquid diets but you contradicted that statement.
My surgeon stated it's based on preference and also where you are having the surgery. My surgeon is performing my surgery in a Circle of Excellence (COE) hospital that requires the two week liquid diet because it has shown better outcomes, reduced surgery time, and less complications. Another hospital system that has their own bariatric program in my state does a one week liquid diet after a week of low calorie, low carb meals with lean protein.
Like any other medical procedure, the pre-op isn't always a one size fits all.