It's to detest the frequent returns on the platform, they can't charge the reverse shipment fees so have settled with platform fee.
For sketchy accounts Myntra will not even create a reverse pickup, and ask the user to ship to their warehouse saying we'll cover your courier charges, again to create friction for user in returning stuff.
They're a business not a charity, most people still buy offline while discounting the personal overheads incurred for the feels of doing stuff.
Flipkart even has started charging offer handling fees of ₹50, over and above their secured packaging fees.
Even tata cliq is charging ₹29 fee per order.
It's an industry wide phenomenon.
These non refundable charges goes a long way to nudge the user not to return stuff rather keep it as it's already delivered and this minute deduction is enough of a swing.