Honestly, I don't like to borrow airtime until 31st night when I couldn't find card to buy and I decided to borrow N100.00 to call someone to send airtime and immediately, N15.00 was deducted leaving N85.00.

Though, it seems very small but it wasn't so before and imagine how many people that are involved in such transaction daily in Nigeria.
Even, the sellers only gets N3.00 profits for each N100.00 sold for Mtn.

Now, my questions are; Is it NCC that authorised and approved that? If yes, how much is the government getting from it? And why so much amount charged for something that most people don't owe for up to 24 hours?

That is 15℅ for any amount borrowed.
Those with knowledge about this should throw more light please.