Why do we add indirect business taxes(sales taxes,excise taxes) to the national income when we compute GDP using the income approach?

Indirect taxes such as sales tax and excise tax do not have any effect on computation of GDP using income approach. In the income approach, GDP is measured as total of all household income which is the sum of  salaries, interests, rent and profit. Indirect taxes do not figure any where in computation of these. They form a component of the price of goods purchased, these do not feature in the total household income at all.

However, taxes such as corporate/income tax need to be subtracted from the gross profit of firms to arrive at their net profit. Generally firms do not include the amount of indirect taxes in computation of gross profit. But if a firm follows an accounting system that includes the amount of indirect taxes in gross profit then these will also have to be subtracted to arrive at net profit.