there isn't a right or a wrong.
we just have effed up broken systems . it doesn't cost Visa any more to process a $500 purchase than a $5 purchase. I expect it's under a dollar to process if there even is a real cost yet Visa charges a %.
some say it's because they are guaranteeing the money but meh.
we pay a fee when buying BTC and the fee is less or more based on the speed. that seems more reasonable.
i think the players in the way things are now have a vested interest in keeping it the same.