Merchants can choose what platform or form of payment with which to transmit and accept currency. Paper, plastic, PayPal, check, wire transfer, or a combo. They can say 'we accept this brand of plastic card but not that brand of plastic card.' They can say 'we won't accept certain denominations of cash.' They could say 'we only accept newer issues of the $100 bill that have authentication features and not older $100 bills.' Nothing wrong with that.
What IS wrong is for merchants to collude together and create their OWN, novel platform of payment transmission, and in direct coordination with the creation of that platform, purposefully drop and/or block other, similar, competing platforms that they had previously accepted.
To shop at Costco you have to get an AmEx card because that's the only form of payment they accept (other than cash.) AmEx and Costco have a partnership. Costco didn't create their OWN card or make you open a banking account or a line of credit with THEM. They made you do it with AmEx.
OK - Not a perfect analogy but here goes:
Studios can release movies on the formats of their choice. Big screen, Netflix, DVD, Blu-Ray, VHS, LaserDisc. Theoretically a studio could release their film with only specific preferred theater chains, or they could decide that they are only going to release Blu-Rays and not DVDs. They could partner with Netflix and make it available there exclusively. Nothing wrong with that.
What WOULD be wrong is if Fox, Disney, and Warner Bros colluded together and created their own 'Red-Ray' disc and 'Red-Ray' player, that would only be able to play Fox, Disney, and WB discs, while simultaneously somehow bricking their previously released BluRay and DVD discs. If Disney, Fox, and WB opened their own nation-wide theater chain, and announced they would not be releasing their films in any other theaters, no one would be balking when theater chain owners filed an anti-trust lawsuit against them.
But WAIT! APPLE IS A MERCHANT! And while they didn't collude with other merchants to create it, they DID create their OWN platform (which of course they accept), and they DON'T accept the competing CurrentC platform in their stores! HAH!
But wait... CurrentC doesn't exist yet, and NFC has existed and been in use for years. Ok, so what if Apple did what MCX is doing? Let's say QR payments came out years ago, and was in use lots of places, including Apple. Then all of a sudden when MCX comes out with CurrentC and lots of people adopt it, Apple blocks/drops QR payments from their stores because 'Hey guys, we created our OWN thing called NFC and Apple Pay, it's not ready yet, but hey we won't have to pay fees to MCX! Sorry but if you want to buy an Apple product with a mobile device, you're going to have to use OUR platform (when it comes out.) What would everyone be saying in reaction to that? maybe...hmm... APPLE TAX!