Indeed, and I hear what you say, BUT was the shotgun threatening HIM? I am playing the devils advocate, I know, but I am pretty sure in stating that having a CCW is NOT a legal reason OR A RESPONSIBILITY to act as a LEO. I do share your feelings, but there are legal ramifications.
One of the biggest problems in the US today is the belief that it is the responsibility of someone else to do something to stop crime. All citizens should be will to help, it is a moral responsibility of all to help stop crime.
Let's see. The guy has a mask on, is waving a shotgun, and is committing a serious armed felony. If I were the customer, I certainly would be in fear for my life in such a situation. Why should I wait until he decides to point it at me and pull the trigger? By then, it would be too late to do anything about it except die.
The threat is real, deadly, and imminent and about anybody in their right mind would be afraid for their life. True, I don't have the LEO expectation to intervene to stop the crime, but I do have the legal right of defending my own life and it is in mortal peril at this point.