That is to say, in the merchant agreement required to accept credit cards, merchants promised to the credit card companies that they would not charge users of credit cards more than cash payers.
As another poster points out, this anti-competitive bullshit was made illegal by law circa 2012.
These kinds of coercive/collusive agreements among a cartel of providers of a particular type of service worry me deeply, especially when a previously-luxury service (e.g., credit cards) becomes semi-necessary infrastructure for operating in the modern world. For more on this type of problem, I encourage you to read about the unbanked [0] and the challenges they face.
Despite how much better they make my life, I also look at the success of Uber, Lyft, AirBnB, and other platforms with worry, precisely because they are in many cases creating semi-necessary infrastructure in the private domain. Individuals can already be banned from Uber [1] and Lyft (and Google [1], etc.) without any legal recourse, which also means by mistake because you are treated as guilty until you can prove your innocence, and innocence is usually impossible to prove.
Here's one (hypothetical and admittedly unlikely) future I worry about: autonomous cars rule the world, 99% owned by four companies (Uber, Lyft, Waymo, ???) that provide Uber-style transportation services. There's no public transport anymore (all outsourced to those four, it's much cheaper than dealing with public transit unions!) and so to get around more than walking distance requires being in the good graces of at least one of those four companies.
Poor John takes a ride in an Uber, gets sick from something he ate in earlier that day, and makes a mess in the car. Uber bans him for this infraction, and shares his name on a ban list (e.g., credit reports) so now John is banned from transportation, and has no recourse (e.g., Google's Gmail bans) -- for the simple mistake of eating undercooked scallops, John is now royally fucked because our infrastructure is outsourced and our society has no say.
I don't expect this future to come about. I expect heavy regulation of these semi-necessary infrastructure companies, and I expect it will come soon. GDPR is one example of this pushback, perhaps not the best example. But I welcome more.
[0]: https://en.wikipedia.org/wiki/Unbanked
[1]: https://www.elliott.org/problem-solved/i-am-banned-by-uber-c...
[2]: http://www.jonasblog.com/my-gmail-account-got-deleted (among many)