Yes, retailers pay the interchange. The Visa people say that the retailer doesn't pay the interchange because it is determined by Visa and MasterCard and, depending on the processor you use, it's through it that you are billed. At the end of the day, the interchange is billed directly to the merchant, including the other fees that, I think, the research attachés mentioned.
Evidence of meeting #27 for Finance in the 40th Parliament, 2nd session. (The original version is on Parliament’s site, as are the minutes.) The winning word was fees.
A recording is available from Parliament.