Suggest you to read this post for detailed explanation on how pledging of stocks for trading F&O works with us.
50% of margin required for F&O position has to come in cash. Whatever money you are short by would attract an interest cost as you would be using the brokers funds.
Some traditional brokers who charge % brokerage waive these charges off as they earn quite a bit on brokerage charges. It makes up for the interest outgo + a lot more for the broker. Check this query on how interest rates are calculated.
As a discount broker, we can’t take this overhead. Hence we charge clients interest when they use our funds.