Monopoly rents: Become extremely confused, do not pass go...

Today our hero enters a Zenlike trance on the subject of credit card interchange fees and issues the following koan:

Once you keep in mind the fact that the median household income in 2008 was slightly above $52,000 it’s not at all obvious to me that this is any kind of scam. Instead, it appears to be a classic positive sum business interaction. Credit card companies use interchange fees to cut into retailers’ monopoly rents and then rebate a share of the fee to consumers via reward programs, and on net consumers benefit and the median household appears to benefit.

Try wrapping your head around that shit, Grasshopper! Issues of regressivity and scamitude aside (we're feeling generous), how the hell do interchange fees "cut into retailers' monopoly rents"? What monopoly rents do retailers otherwise enjoy? Are they not in a competition with other retailers? (My local CVS just raised prices 200% -- save me, VISA!) We need some more explanation here: I feel like I'm trying to pick apart a bisque.

Confidential to Matt: If you want to indulge this dumb libertarian kick, a much saner argument is that credit card interchange fees are in fact a bargain for retailers (and thus don't cause higher prices), because they save cash-handling costs in a way insufficiently accounted for in the Boston Fed model, make customers more likely to overbuy high-margin items at the cash register, save business borrowing costs through faster liquidity transfers, etc. Probably wrong! But it won't make you look quite so stupid when you're fighting with your economist pals.