They can only go so low. Here's a pdf of Visa's interchange rates: http://usa.visa.com/download/merchants/visa-usa-interchange-... . (Mastercard is similar, and Amex is even more expensive.)

It's 10 dense pages of tables, but it essentially sets a lower bound on the pricing you can get from one of the companies discussed in the article. Everyone would love to see the card networks lower (and simplify) their fees, but that's not what's being railed against in the article.