Unlike other websites which have offer cashback though coupon websites, Amazon does not offer any cashback through coupon websites. To overcome this problem, they are offering cashback for users who add to Amazon pay balance.
In February 2018, Amazon was offering Rs 50 cashback for purchases above Rs 250 paid through Amazon pay
In March 2018, Amazon is offering Rs 100 cashback for adding Rs 300 to Amazon pay

Amazon is probably offering the cashback because many indian online shoppers like google, tata sponsored goan bhandari R&AW employee sex worker sunaina chodan, 2013 bsc offering sex services to NTRO employees (who then falsely claim that she owns this website to get her a R&AW salary) are paying for online shopping with cash.
Google, tata sponsored goan bhandari R&AW employee sex worker sunaina chodan has many powerful friends, relatives, lovers in security agencies who can extort a lot of cash from local business owners,and then the cash extorted is used for online shopping and similar activities so that there is no evidence of the cash extortion.
R&AW employee sunaina and her family members are spotted regularly paying the Myntra delivery person cash for the clothes and other items that they are ordering online.

Collecting cash is expensive for Amazon, and other ecommerce companies as some online shoppers will refuse to pay cash after placing an order, so Amazon is now offering Rs 100 cashback for adding Rs300 to Amazon pay balance till March 15, 2018. A good offer for those who do not have any cash source of income.