Conversely, what's to prevent Apple from adding NFC to *all* its devices? The iPad with Square dongle is a very popular combination with small retailers - once the next iPad Air has NFC, I'm sure it will be able to act as a payment receiver. Heck, there is nothing preventing Apple from enhancing existing iPads and iPhones to make payments to each other via Bluetooth LE.
This isn't just a matter of communication. There are some very serious security considerations. Just imagine what would happen if security was breached on this, that would be absolutely desastrous.
For example, Apple Pay will never, ever, run on an iPhone 5s or earlier. That's because the iPhone 6 has one specific chip inside that does all the security related things in Apple Pay, which has been either heavily examined by the banks or was actually developed by the banks (I think it is the latter).
Apple must guarantee that no iPhone 6, even if it is jailbroken, hacked with malware, full of viruses, will make credit card payments without the consent of the owner. They can't do that with pre-iPhone 6 hardware.
----------
It'll be interesting to see how the credit card companies adapt to Apple pay. I've heard the NFC token-based authentication used by Apple pay is much more secure than the chip method. If so, the credit card companies are likely to push for Apple pay, as that'll help decrease fraud (and thereby increase credit card companies' profits).
Here's what makes Apple Pay safer: Nobody, including the merchant, or any employee of the merchant, ever finds out your credit card number.
Fraud is currently quite simple in the USA if a crook has your credit card number. Apple Pay cannot prevent that, because the crook doesn't use Apple Pay! With Chip&PIN or other security measures that will soon be introduced, it's harder to go to a shop with a fake credit card and commit fraud, but knowing your credit card number I can still buy things online fraudulently (and Apple Pay cannot prevent it, because the crook doesn't use Apple Pay).
However, all that fraud requires that some crook gets hold of your credit card number first, most likely through a crook working in some genuine, honest store who takes your credit card. And if you are using Apple Pay, they can't get hold of the number!