Third World countries? This is similar to the countries that had minimal wired rural telephone service and only achieved "universal" phone service with the advent of cellular technology.
Developing economies have a tendency to skip expensive intermediate technologies (like dedicated card readers) and implement lower-cost/higher-tech solutions sooner. In rich countries banks and card processors can force their merchants to use higher-cost terminal systems (that they sell to the merchants). There's more vendor push-back in developing economies, for obvious reasons. So if the banks/processors want the big bucks (skimming a percentage of every transaction) that they get from near-universal implementation, they may have to forgo the ancillary revenues from selling card readers/terminals in order to gain that vendor acceptance.
This is another step towards the "death of cash." Governments prefer all financial transactions be visible/traceable (and potentially taxable). Banks prefer to not handle/safeguard cash. All but mom-and-pop businesses prefer that their employees not handle cash. So if the smart phones already in the hands of nearly the entire population can be used to reduce the use of cash in business... it's going to happen.
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Sometimes Apple uses its hardware/software platforms to create new services, sometimes they add new features to the for the sake of encouraging wider adoption of its hardware (once they have the hardware, they have an opening for selling unrelated services). I think this will turn out to be the latter - selling more iPads and iPhones to merchants, field service workers (plumbers, appliance repair, etc.).
Still... While for now this is all about accepting tap-to-pay cards, it will undoubtedly extend to receiving Apple Pay payments as well. Why would an Apple Pay user accustomed to tapping their Watch or iPhone on a payment terminal want to go back to pulling a card out of their wallet? Of course Apple Pay has been delivering a revenue stream to Apple since Day One.
Not only does this sell more devices to business, but as Apple learned long ago in the personal computer business, consumers tend to buy the same equipment they use at work for their at-home use. MS-DOS/Windows PCs at work, the same at home.
Altogether, Apple's control over software and hardware means that when they do offer a new capability like this, they're able to roll it out across several generations of product, not just the latest. Rapid adoption can be critical - an overnight software roll-out to an already-installed hardware base can translate to a one- or two-year head start over the fractured Android market. And once merchants are accustomed to using iPhones/iPads for direct payment, they're not going to be in a hurry to switch platforms.
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Getting back to government for a minute. Despite various government attempts (pushed by Apple's competitors, of course) to open Apple's platform, Apple's arguments for security/privacy tend to be supported by banks and other financial institutions. So an Apple that is a government ally in reducing/killing the cash economy is unlikely to get major regulatory pushback on maintaining a closed hardware/software environment. If Treasury says to Consumer Protection, "We need this to be locked down," Consumer Protection will start advocating the closed system as a form of consumer protection.