The case studies in TFA are all places that deal with extremely high volume over very short time periods—amusement parks, zoos, and sporting events. Many of their points of sale are also mobile (food carts or someone walking around with a tray). Both the volume and the mobility make cash much more difficult to manage than a phone with a card reader, so they've likely done the math and determined that the interchange fees more than pay for the logistical improvements.
Remember when Chipotle got in trouble for rounding 24¢ and 26¢ to 25¢, to save time making change? [1] It was not unfair on average, and it surely sped things up, but they stopped because it's technically not correct.
I can see if being considered unfair if it's a single business, but I do think that it's at least a little funny, considering we have penny rounding (to the nearest nickel) here in Canada since geting rid of the penny. Now it's by default, everywhere. And as someone that uses cash for all in-person purchases, it's actually really, really nice
