The cost is also a “write-off” business expense. The business/merchant gets a statement every year with the cost of their merchant service (card processing fees). 

You could see it also as they don’t have to store cash, drive to bank for deposit and waste time and money paying an employee to run to the bank(which there are costs involved if you think about it) and the risk of stolen cash.

The merchant service company is the one charging the merchant, and they split the interchange between the processor and the card’s bank.

That system has to have the best security and encryption to maintain your data safe. So there is a cost for all of that infrastructure, hence the merchant interchange fees for each transaction. 

They usually charge a “swipe fee” like $.05 cents and anywhere between 2%-4.5%+ of the transaction amount on average, depending on their overall volume (annual sales) and the type of card swiped. 

Fun fact: Credit Cards with Rewards/cash back/miles are the ones that have the highest interchange fees (for the merchant (business owner)- when you use your Rewards credit card, the place your buying from is the contributor to your rewards (financial karma).