Lots of inaccurate information on the backend of Apple Pay. Visa \ MC \ Amex \ UPI \ etc. are called schemes. These schemes are actually global. This is exactly why ApplePay actually does work at almost any contactless merchant terminal globally today.
The limitation is not on the merchant side (the store and provider (know as the acquiring institution) that process the transaction), but the issuer (typically banks that "issue" cards to consumers). The total fee to the merchant has NOT gone up at all. What Apple has done is to negotiate taking a small slice (15 basis points), out of the issuer revenue per transaction (Interchange Fees). So effectively the banks that issue the cards agree to give Apple a slice of their profit. The business case is that this will be made back by the banks primarily through lower fraud rates and higher volume. The schemes (Network Fees) and acquiring institutions (Processing Fees) who are the other parties that take a cut of the overall transaction fees are unaffected by Apple Pay. To them it is just another transaction.
So if you onboard an issuer to ApplePay, Apple then enables the card to be added to Apple Pay (via Passbook) and also enables on the backend a interface with the bank to collect their ~15 basis points per transaction. Once a card is onboard, the schemes (again Visa, MC, etc.) see it as just another transaction (although one that uses tokenization which the schemes have supported for years now).
See this breakdown of the transaction fees...
https://bluenotes.anz.com/media/145510/Sample_apple_pay_trans.png
So in the UK example, if I have Apple Pay enabled and loaded with a supported card from one of the partner US issuers, it will work fine at any merchant terminal that supports the standard scheme contactless tokenization (almost all contactless stations globally). If however I want to use a card issued by a UK issuer (i.e. RBS, etc.) Apple will not allow it to be added because the terms and interface to the UK issuers have not been established yet (i.e. no way for Apple to take their small transaction fee). This is actually no different that let's say a small saving and loan issuer in the US that is not signed up yet. You can't use that card until the bank enters an agreement to enable taking the cut.
The acquirers (i.e merchants) nor the schemes (i.e. Visa, MC) actually have anything to do with the rollout. Apple just needed to work with them initially to ensure ApplePay complied with the global schemes contactless tokenization standards.
This tie up with the issuing banks is what sets Apple Pay completely apart from what Google and others are trying to do. It is fundamentally a different business model, and this is actually why Apple Pay is set to have a big impact. It has not nearly as much to do with the front end technology where Apple Pay and Goole wallet may appear to be very similar.
Hope this helps clear all this up. Much misinformation out there on how all this works. Lets just say I work in the industry so I could provide some clarity.