Aaaactually...
It depends on how the retailer's POS system works. If all the POS system expects is the PIN pad to turn around and say "Ok!" (if they even have an integrated POS system and not just a separate, standalone PIN pad into which they key an amount manually), then sure, it's a software switch to be flicked at the payment processor who owns/maintains the pinpad (in Australia, it's gonna be controlled/owned by a bank, Tyro or PaymentExpress generally speaking, and leased to the store).
If, however, you're talking about a large, long term business in retail, then chances are that their backend systems have all sorts of complexities around supported card types, authorisation/accounting/reconciliation of those card types, and how refunds are handled... they might need significant work to support tokenisation when matching transactions from the bank the next day, or when needing to refund someone months down the track etc.. This is especially true for businesses that went from reading magstripes on keyboard attachments etc. to needing to integrate PIN pads which did the auth direct to the bank themselves and only return partial details (which is more secure, but not how the POS systems were designed).
Either way, at least this article explains why only AMEX has Apple Pay in Australia :)
Biting the hand that feeds IT
