So let me explain a bit.
An EMI is basically a loan. The interest charged goes into the pocket of the creditor.
A no-cost EMI seems to be a zero interest loan. But here's the thing, RBI doesn't recognise zero interest loans. Interest needs to be charged and the banks also want that.
So retailers or the brand itself passes off the savings they would have given to you, to the bank instead. So for example: Sony would have provided 15k additional discount to the customer but instead they are providing this to the bank to cancel out the interest.
Also, no-cost EMI does charge processing fee and GST extra.
Now why don't brands just directly give customers the discount? Well, they get kickback for the traffic they drive on payment network like Visa, MasterCard or the bank's network (more EMI = more monthly processing fee).
So EMI (no cost or not) only makes sense when you are getting decent cashback. But then, isn't the bank ultimately losing money?
Well, the biggest reason behind all this hoopla is to get you hooked on using credit/debit cards loans, on using the money you don't actually have. And then in the case of late payment or default, they suck huge interest out of you. Enough % of people either forget the payments or pay late or pay just the minimum amount or default for the banks to make great profit out of it.
I personally have seen many people just paying the minimum due amount, thinking they won't be charged anything, only to realise that it only saves you from the late payment penalty but you still have to pay interest.
And that's why you get cashback only on EMI.
And yes, cashback only offered on 6 month+ EMI. But the cashback is quite good, makes sense to buy and then pay off the credit due immediately.
Sorry for going a bit off topic.