Apple didn't mess up anything. All Apple does is send the encrypted card information to the POS via the NFC transmission. From there, it is all up to the POS, and the credit card processor (whatever company that the POS system is setup to process with. This can be Vantiv, Mercury, FirstData, TSYS, etc). If declines are happening, it's coming from the processor or from the bank. Apple (smartly) doesn't have anything to do with the actual "processing" of this.
Credit card processing is actually extremely complex when you dig deep into it. This is the industry that I'm involved with. Most people have no idea that their credit cards are sent through multiple different companies and processors for one transaction to go through. Sometimes as many as six different companies are involved when it comes to processing a single transaction.
It's very complex, and I'm sure people who have no idea how credit card processing works will probably comment in this thread as if they know.