A merchant account is the power to make any credit/debit card charge given the card details.

This is an incredible power. The power to effectively drain the bank account of every customer who has ever done business with you.

For credit card transactions the basic mechanism for making a payment is fundamentally equivalent to handing over a signed stack of blank checks to the merchant and having them fill in all the details.

Yes, this highlights how massively broken the system is. However, it also highlights why it's necessary to avoid every Joe on the street having a merchant account.