I have explained why this is important in this article on QR codes and NFC tags which includes a video that makes the point quite vividly: You should not let your hardware act on the instructions embedded in a QR code of NFC tag without first knowing what those actions are. The AT&T code scanner for iPhone is set up to do that. Other scanners also have that ability but do not behave that way by default.
I have bashed AT&T for poor wireless products and service on numerous occasions, but I believe in praise where praise is due. Security has long been a priority at AT&T. Over the years I have trained thousands of AT&T employees on everything from server security to security in the workplace. So I was happy to see their QR code reader was designed right.