It's all show business. Refusing to endorse the fairness cream commercial and then going viral for that noble act is more lucrative than earning ₹ 2 crores for it.
Rejecting the offer for product promotion is different from using those products for cosmetic purposes. In the industry (film) it is a known fact that all make-up professionals carry imported beauty products in a vanity box. The industry of vanishing cream and other skin shining products could not capture markets in African Nations where black is the reigning queen. Riding on a popular wave is the technique of marketing science. The celebrities by promoting products makes first money and invariably never consume it for themselves.