Income tax on cashbacks.

I recently read an article that cumulative cashbacks above 50000 is taxable. How is that an income? It is a reduction in our expenses out of our income already chargeable to tax.

1.Income from salaries- Salary received by an employee from employer.
2.Income from house property- Rent received on letout property
3.Profits and gains of business or profession- Net proift for business or profession.
4.Capital Gains- profit on sale of capital assets.
5.Income from Other Sources- Interest,lottery,winnings from horse races,gift,agri income etc.

Now.. Under which head is a reduction of expense calculated?